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
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Consolidated Statements of Income:      
Revenue $ 60,007 $ 30,407  
Taxes, other than income taxes 144 55  
Income taxes 10,820 9,638  
Net income $ 43,791 $ 19,259  
Net income per Sub-share Certificate (in usd per share) $ 5.60 $ 2.43  
ASSETS      
Accrued receivables $ 27,644   $ 17,773
Deferred tax liability (114)   (114)
Liabilities and Capital:      
Unearned revenue 8,932   8,364
Other taxes payable     0
Net proceeds from all sources $ 111,333   105,902
Scenario, Previously Reported      
Consolidated Statements of Income:      
Revenue   $ 24,229  
Taxes, other than income taxes   660  
Income taxes   7,228  
Net income   $ 14,886  
Net income per Sub-share Certificate (in usd per share)   $ 1.88  
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
Accounting Standards Update 2014-09 | Restatement Adjustment      
Consolidated Statements of Income:      
Revenue   $ 6,178  
Taxes, other than income taxes   (605)  
Income taxes   2,410  
Net income   $ 4,373  
Net income per Sub-share Certificate (in usd per share)   $ 0.55  
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