Convertible Notes (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Debt Instrument [Line Items]
Convertible notes, short-term	$ 0	$ 164,047
Convertible notes, long-term	112,316	109,629
1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Principal amount of convertible notes	138,000
Convertible Senior Notes
Debt Instrument [Line Items]
Principal amount of convertible notes	138,000	310,500
Unamortized discount	(25,684)	(36,824)
Total convertible notes	112,316	273,676
Convertible notes, short-term	0	164,047
Convertible notes, long-term	112,316	109,629
Convertible Senior Notes | 5% Convertible Senior Notes due 2014
Debt Instrument [Line Items]
Convertible notes stated interest rate (as a percent)	5.00%	5.00%	5.00%
Principal amount of convertible notes	0	172,500
Unamortized discount	0	(8,453)
Total convertible notes	0	164,047
Convertible Senior Notes | 1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Convertible notes stated interest rate (as a percent)	1.125%	1.125%	1.125%
Principal amount of convertible notes	138,000	138,000
Unamortized discount	(25,684)	(28,371)
Total convertible notes	$ 112,316	$ 109,629