Debt - Schedule of Convertible Senior Notes (Details)		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 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	(22,132,000)	(22,132,000)	(26,464,000)
Total	552,861,000	552,861,000	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	0.0530969	53.0969
Remaining Amortization Period		1 year
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 6 months 17 days
Principal Amount	$ 345,000,000	$ 345,000,000	$ 345,000,000	$ 345,000,000
Convertible debt, fair value disclosures				$ 324,252,000