Debt - Schedule of Convertible Senior Notes (Details)		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 16, 2017 USD ($)	Jan. 31, 2017 USD ($)	Jul. 31, 2014 USD ($)
Convertible Notes Payable [Abstract]
Principal Amount	$ 574,993,000	$ 574,993,000	$ 575,000,000
Net unamortized fair value adjustment	(19,912,000)	(19,912,000)	(26,464,000)
Total	555,081,000	555,081,000	548,536,000
Debt instrument, principal amount used for conversion	$ 1,000	$ 1,000
2019 Convertible Notes | 3.00% Convertible Senior Note
Convertible Notes Payable [Abstract]
Coupon Rate	3.00%	3.00%				3.00%
Effective Rate	4.92%	4.92%
Conversion Rate	0.0537294	53.7294
Remaining Amortization Period		9 months
Principal Amount	$ 229,993,000	$ 229,993,000	230,000,000			$ 230,000,000
Convertible debt, fair value disclosures				$ 223,185,000
2022 Convertible Notes | 3.50% Convertible Senior Note
Convertible Notes Payable [Abstract]
Coupon Rate	3.50%	3.50%			3.50%
Effective Rate	5.12%	5.12%
Conversion Rate	0.0437694	43.7694
Remaining Amortization Period		3 years 3 months 19 days
Principal Amount	$ 345,000,000	$ 345,000,000	$ 345,000,000		$ 345,000,000
Convertible debt, fair value disclosures				$ 324,252,000