Debt - Debt issuance costs (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 855,000	$ 800,000
Debt issuance costs, net	(7,700)	(8,145)
Long-term debt, net	847,300	791,855
Senior Notes | January 2022 Notes
Debt Instrument [Line Items]
Long-term debt	450,000	450,000
Debt issuance costs, net	(3,743)	(3,987)
Long-term debt, net	446,257	446,013
Senior Notes | March 2023 Notes
Debt Instrument [Line Items]
Long-term debt	350,000	350,000
Debt issuance costs, net	(3,957)	(4,158)
Long-term debt, net	346,043	345,842
Senior Secured Credit Facility | Line of Credit
Debt Instrument [Line Items]
Long-term debt	55,000	0
Debt issuance costs, net	0	0
Long-term debt, net	55,000	0
Senior Secured Credit Facility | Line of Credit | Other Assets
Debt Instrument [Line Items]
Debt issuance costs related to line of credit arrangements	$ 5,700	$ 6,000