Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Adoption of New Accounting Standards (Details)

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Summary of Significant Accounting Policies - Adoption of New Accounting Standards (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating income $ 260,834 $ 144,947 $ 62,911
Other income (expense) $ 916 114 (15)
As reported in prior year      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating income   119,776 55,058
Other income (expense)   84 29
Accounting Standards Update 2017-07 | Retrospective adjustment      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating income   25,171 7,853
Other income (expense)   $ 30 $ (44)