Debt (Details) - USD ($) $ in Thousands, shares in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014		Sep. 30, 2014
Convertible Notes
Less current portion	$ 5	$ 85,640	[1]
Convertible senior notes noncurrent	623,928	603,478	[1]
Carrying Amount
Convertible Notes
Convertible senior notes, total	623,933	689,118
Less current portion	5	85,640
Convertible senior notes noncurrent	623,928	603,478
Fair Value
Convertible Notes
Convertible senior notes, total	$ 1,622,164	1,893,035
Convertible Senior Notes 4.75 Percent Due 2015
Convertible Notes
Interest rate of debt (as a percent)	4.75%			4.75%
Aggregate principal amount of notes exchanged	$ 90,800
Common stock issued in exchange of notes (in shares)	10.4
Convertible Senior Notes 4.75 Percent Due 2015 | Carrying Amount
Convertible Notes
Convertible senior notes, total	$ 5	85,640
Convertible Senior Notes 4.75 Percent Due 2015 | Fair Value
Convertible Notes
Convertible senior notes, total	57	755,143
Convertible Senior Notes 4.75 Percent Due 2015 | Director
Convertible Notes
Aggregate principal amount of notes exchanged	$ 43,300
Common stock issued in exchange of notes (in shares)	4.9
Convertible Senior Notes 0.375 Percent Due 2018 and Convertible Senior Notes 1.25 Percent Due 2020
Convertible Notes
Number of days within 30 consecutive trading days in which the price of the entity's common stock must exceed the conversion price for the notes to be converted	20 days
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible	30 days
Percentage of the closing sales price of common stock that the conversion price must exceed in order for the notes to be convertible (as a percent)	130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period	5 days
Number of consecutive trading days before five consecutive business days during the note measurement period	5 days
Conversion ratio, principal amount of note	1,000
Percentage of the trading price to the product of the last reported sale price of the common stock and the conversion rate, maximum (as a percent)	98.00%
Convertible Senior Notes 0.375 Percent Due 2018
Convertible Notes
Interest rate of debt (as a percent)	0.375%
Convertible Senior Notes 0.375 Percent Due 2018 | Carrying Amount
Convertible Notes
Convertible senior notes, total	$ 325,506	314,752
Convertible Senior Notes 0.375 Percent Due 2018 | Fair Value
Convertible Notes
Convertible senior notes, total	$ 808,125	560,156
Convertible Senior Notes 1.25 Percent Due 2020
Convertible Notes
Interest rate of debt (as a percent)	1.25%
Convertible Senior Notes 1.25 Percent Due 2020 | Carrying Amount
Convertible Notes
Convertible senior notes, total	$ 298,422	288,726
Convertible Senior Notes 1.25 Percent Due 2020 | Fair Value
Convertible Notes
Convertible senior notes, total	$ 813,982	$ 577,736

[1] The condensed consolidated balance sheet at December 31, 2014 has been derived from the audited financial statements at that date.