Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended										12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015		Mar. 31, 2015	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data (Unaudited) [Abstract]
Revenue		$ 51,571	$ 49,121	$ 120,428		$ 62,583	$ 84,861		$ 44,063	$ 57,076	$ 28,161	$ 283,703	$ 214,161	$ 147,285
Operating expenses		114,511	97,259	75,782		71,913	74,781		65,556	63,726	57,828	359,465	261,891	198,951
Loss from operations		(62,940)	(48,138)	44,646		(9,330)	10,080		(21,493)	(6,650)	(29,667)	(75,762)	(47,730)	(51,666)
Net income (loss)		$ (71,433)	$ (35,776)	$ 35,648		$ (16,717)	$ 31,053		$ (26,676)	$ (12,081)	$ (31,280)	$ (88,278)	$ (38,984)	$ (60,644)
Basic net income (loss) per share (in dollars per share)	[1]	$ (0.59)	$ (0.30)	$ 0.30		$ (0.14)	$ 0.26		$ (0.23)	$ (0.10)	$ (0.27)
Diluted net income (loss) per share (in dollars per share)	[1]	(0.59)	(0.30)	$ 0.29	[2]	(0.14)	$ 0.25	[3]	(0.23)	(0.10)	(0.27)
Income (Numerator) [Abstract]
Income available to common shareholders				$ 35,648			$ 31,053
Dilutive effect of 2 3/4 percent notes				1,047
Income available to common shareholders, plus assumed conversions				$ 36,695			$ 31,053
Shares (Denominator) [Abstract]
Shares used in computing basic net income per share (in shares)				119,742			118,223
Effect of Diluted Securities [Abstract]
Shares issuable related to our ESPP				4			9
Shares issuable related to our 2 3/4 percent notes (in shares)				3,683
Shares used in computing diluted net income per share (in shares)				127,779			122,839
Per-Share Amount [Abstract]
Basic net income per share (in dollars per share)	[1]	(0.59)	(0.30)	$ 0.30		(0.14)	$ 0.26		(0.23)	(0.10)	(0.27)
Diluted net income per share (in dollars per share)	[1]	$ (0.59)	$ (0.30)	$ 0.29	[2]	$ (0.14)	$ 0.25	[3]	$ (0.23)	$ (0.10)	$ (0.27)
Convertible Senior Notes [Member] | 1 Percent Convertible Senior Notes [Member]
Convertible Senior Notes [Abstract]
Interest rate on convertible senior notes		1.00%					1.00%					1.00%	1.00%
Convertible Senior Notes [Member] | 2 3/4 Percent Convertible Senior Notes [Member]
Convertible Senior Notes [Abstract]
Interest rate on convertible senior notes		2.75%					2.75%					2.75%	2.75%	2.75%
Stock Options [Member]
Effect of Diluted Securities [Abstract]
Shares issuable related to stock-based compensation (in shares)				3,974			4,189
Restricted Stock Awards [Member]
Effect of Diluted Securities [Abstract]
Shares issuable related to stock-based compensation (in shares)				376			418

[1] We computed net income (loss) per share independently for each of the quarters presented. Therefore, the sum of the quarterly net income (loss) per share will not necessarily equal the total for the year.

[2] For the three months ended June 30, 2015, we had net income. As a result, we computed diluted net income per share using the weighted-average number of common shares and dilutive common equivalent shares outstanding during the period. For the three months ended June 30, 2015, the calculation excludes the 1 percent notes because the effect on diluted earnings per share would be anti-dilutive.

[3] For the three months ended December 31, 2014, we had net income. As a result, we computed diluted net income per share using the weighted-average number of common shares and dilutive common equivalent shares outstanding during the period. For the three months ended December 31, 2014, the calculation excludes the 1 percent and 2.75 percent convertible senior notes because the effect on diluted earnings per share would be anti-dilutive.