Debt (Details) - USD ($) $ in Thousands, shares in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014		Jun. 30, 2014
Convertible Notes
Less current portion	$ 42,940	$ 85,640	[1]
Convertible senior notes noncurrent	617,025	603,478	[1]
Carrying Amount
Convertible Notes
Convertible senior notes, total	659,965	689,118
Less current portion	42,940	85,640
Convertible senior notes noncurrent	617,025	603,478
Carrying value of notes	659,965	689,118
Fair Value
Convertible Notes
Convertible senior notes, total	2,059,297	1,893,035
Carrying value of notes	$ 2,059,297	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	$ 47,100
Common stock issued in exchange of notes (in shares)	5.4
Convertible Senior Notes 4.75 Percent Due 2015 | Carrying Amount
Convertible Notes
Convertible senior notes, total	$ 42,940	85,640
Carrying value of notes	42,940	85,640
Convertible Senior Notes 4.75 Percent Due 2015 | Fair Value
Convertible Notes
Convertible senior notes, total	519,217	755,143
Carrying value of notes	$ 519,217	$ 755,143
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	$ 321,876	$ 314,752
Carrying value of notes	321,876	314,752
Convertible Senior Notes 0.375 Percent Due 2018 | Fair Value
Convertible Notes
Convertible senior notes, total	766,875	560,156
Carrying value of notes	$ 766,875	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	$ 295,149	288,726
Carrying value of notes	295,149	288,726
Convertible Senior Notes 1.25 Percent Due 2020 | Fair Value
Convertible Notes
Convertible senior notes, total	773,205	577,736
Carrying value of notes	$ 773,205	$ 577,736

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