Long-Term Debt - Convertible Senior Notes Due 2032 (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 20, 2018	Mar. 31, 2012	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	May 04, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal amount			$ 502,195		$ 502,195
Repurchase of convertible debt					60,362	$ 0
Loss on extinguishment of long-term debt			(76)	$ (397)	(1,181)	$ (397)
Convertible Senior Notes Maturing March 2032 (Redeemed May 2018)
Debt Instrument [Line Items]
Principal amount		$ 200,000	$ 0		$ 0			$ 60,115
Interest rate (as a percent)		3.25%
Maturity date		Mar. 15, 2032
Repurchased principal amount	$ 59,300						$ 800
Repurchase of convertible debt	59,500
Payments for fees	200
Loss on extinguishment of long-term debt	$ (200)