Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Adoption of ASU 2017-07 (Details)

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Summary of Significant Accounting Policies - Adoption of ASU 2017-07 (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Operating income $ 65,505 $ 30,379 $ 119,986 $ 59,269
Other income   6   13
As reported in prior year        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Operating income   24,732   46,836
Other income   10   19
Retrospective Adjustment | Accounting Standards Update 2017-07        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Operating income   5,647   12,433
Other income   $ (4)   $ (6)