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 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Condensed Consolidated Statements of Income:          
Revenue $ 73,844 $ 32,293 $ 133,851 $ 62,700  
Taxes, other than income taxes 113 62 257 117  
Income taxes 13,162 10,035 23,982 19,673  
Net income $ 52,503 $ 20,350 $ 96,294 $ 39,609  
Net income per Sub-share Certificate (in usd per share) $ 6.73 $ 2.58 $ 12.33 $ 5.02  
ASSETS          
Accrued receivables $ 35,664   $ 35,664   $ 17,773
Deferred tax liability (114)   (114)   (114)
Liabilities and Capital:          
Unearned revenue 11,313   11,313   8,364
Other taxes payable         0
Net proceeds from all sources $ 153,510   $ 153,510   105,902
As reported in prior year          
Condensed Consolidated Statements of Income:          
Revenue   $ 27,349   $ 51,577  
Taxes, other than income taxes   762   1,422  
Income taxes   8,030   15,258  
Net income   $ 16,711   $ 31,597  
Net income per Sub-share Certificate (in usd per share)   $ 2.12   $ 4.00  
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   $ 4,944   $ 11,123  
Taxes, other than income taxes   (700)   (1,305)  
Income taxes   2,005   4,415  
Net income   $ 3,639   $ 8,012  
Net income per Sub-share Certificate (in usd per share)   $ 0.46   $ 1.02  
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