Convertible Notes (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Convertible Notes
Convertible senior notes	$ 619,893	$ 589,981
Total debt	749,800
Annual maturities of Convertible Notes
2018	375,000
2020	374,800
Total debt	749,800
Carrying Amount
Convertible Notes
Convertible senior notes	619,893	675,167
Total debt	619,893	675,167
Less current portion		85,186
Long-term debt, noncurrent	619,893	589,981
Annual maturities of Convertible Notes
Total debt	$ 619,893	$ 675,167
4.75% Convertible Senior Notes due 2015
Convertible Notes
Interest rate of debt (as a percent)	4.75%	4.75%	4.75%
4.75% Convertible Senior Notes due 2015 | Carrying Amount
Convertible Notes
Convertible senior notes		$ 85,186
0.375% Convertible Senior Notes due 2018
Convertible Notes
Interest rate of debt (as a percent)	0.375%	0.375%	0.375%
0.375% Convertible Senior Notes due 2018 | Carrying Amount
Convertible Notes
Convertible senior notes	$ 324,031	$ 307,978
1.25% Convertible Senior Notes due 2020
Convertible Notes
Interest rate of debt (as a percent)	1.25%	1.25%	1.25%
1.25% Convertible Senior Notes due 2020 | Carrying Amount
Convertible Notes
Convertible senior notes	$ 295,862	$ 282,003