Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Details)

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Fair Value Measurements (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value on Non-Recurring Basis        
Gross carrying value $ 750,757   $ 750,757  
Amount of transfers of assets and liabilities measured on a recurring and non-recurring basis between Levels 1, 2 and 3 of the fair value hierarchy 0 $ 0 0 $ 0
Senior Notes 5.625%        
Fair Value on Non-Recurring Basis        
Gross carrying value 400,000   400,000  
Fair Value on Non-Recurring Basis | Level 1 fair value measurement | Senior Notes 5.625%        
Fair Value on Non-Recurring Basis        
Fair value $ 390,000   $ 390,000