Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Adoption of ASU 2014-09 (Details)

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Summary of Significant Accounting Policies - Adoption of ASU 2014-09 (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Condensed Consolidated Statements of Income:          
Revenue $ 73,168 $ 51,977 $ 207,019 $ 114,677  
Taxes, other than income taxes 130 66 387 183  
Income taxes 12,433 16,322 36,415 35,995  
Net income $ 50,762 $ 33,002 $ 147,056 $ 72,611  
Net income per Sub-share Certificate (in dollars per share) $ 6.52 $ 4.20 $ 18.86 $ 9.22  
ASSETS          
Accrued receivables $ 39,615   $ 39,615   $ 17,773
Deferred tax liability (114)   (114)   (114)
Liabilities and Capital:          
Unearned revenue         8,364
Other taxes payable         0
Net proceeds from all sources $ 195,805   $ 195,805   105,902
As reported in prior year          
Condensed Consolidated Statements of Income:          
Revenue   $ 42,476   $ 94,054  
Taxes, other than income taxes   797   2,219  
Income taxes   12,687   27,945  
Net income   $ 26,405   $ 58,002  
Net income per Sub-share Certificate (in dollars per share)   $ 3.36   $ 7.37  
ASSETS          
Accrued receivables         18,206
Deferred tax asset         6,992
Liabilities and Capital:          
Unearned revenue         41,375
Other taxes payable         433
Net proceeds from all sources         79,997
Retrospective Adjustment | Accounting Standards Update 2014-09          
Condensed Consolidated Statements of Income:          
Revenue   $ 9,501   $ 20,623  
Taxes, other than income taxes   (731)   (2,036)  
Income taxes   3,635   8,050  
Net income   $ 6,597   $ 14,609  
Net income per Sub-share Certificate (in dollars per share)   $ 0.84   $ 1.85  
ASSETS          
Accrued receivables         (433)
Deferred tax liability         (7,106)
Liabilities and Capital:          
Unearned revenue         (33,011)
Other taxes payable         (433)
Net proceeds from all sources         $ 25,905