Fair Value Measurements - Fair Value of Financial Instruments Not Recorded at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt	$ 1,373,638	$ 1,385,632
Fair Value | Senior notes | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt	535,544	745,943
Fair Value | Revolving credit facilities and term loans | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt	838,094	639,689
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt	1,365,638	1,364,429
Carrying Value | Senior notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt	524,950	723,867
Carrying Value | Revolving credit facilities and term loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt	$ 840,688	$ 640,562