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
Mar. 31, 2018
Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating income $ 54,481 $ 28,890
Other income   7
Scenario, Previously Reported    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating income   22,104
Other income   9
Accounting Standards Update 2017-07 | Restatement Adjustment    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating income   6,786
Other income   $ (2)