Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended											12 Months Ended
		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015		Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data (Unaudited) [Abstract]
Revenue		$ 160,349		$ 110,927		$ 38,470	$ 36,874	$ 51,571	$ 49,121	$ 120,428		$ 62,583	$ 346,620	$ 283,703	$ 214,161
Operating expenses		119,194		94,819		87,397	91,526	114,511	97,259	75,782		71,913	392,936	359,465	261,891
Income (loss) from operations		41,155		16,108		(48,927)	(54,652)	(62,940)	(48,138)	44,646		(9,330)	(46,316)	(75,762)	(47,730)
Net income (loss)		$ 25,865		$ 7,351		$ (56,855)	$ (62,917)	$ (71,433)	$ (35,776)	$ 35,648		$ (16,717)	$ (86,556)	$ (88,278)	$ (38,984)
Basic net income (loss) per share (in dollars per share)	[1]	$ 0.21		$ 0.06		$ (0.47)	$ (0.52)	$ (0.59)	$ (0.30)	$ 0.30		$ (0.14)
Diluted net income (loss) per share (in dollars per share)	[1]	$ 0.21	[2]	$ 0.06	[3]	(0.47)	(0.52)	(0.59)	(0.30)	$ 0.29	[4]	(0.14)
Income (Numerator) [Abstract]
Income available to common shareholders		$ 25,865		$ 7,351						$ 35,648
Dilutive effect of 2 3/4 percent notes										1,047
Income available to common shareholders, plus assumed conversions		$ 25,865		$ 7,351						$ 36,695
Shares (Denominator) [Abstract]
Shares used in computing basic net income per share (in shares)		121,340		120,989						119,742
Effect of Diluted Securities [Abstract]
Shares issuable related to our ESPP		21		58						4
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)		123,953		123,378						127,779
Per-Share Amount [Abstract]
Basic net income (loss) per share (in dollars per share)	[1]	$ 0.21		$ 0.06		(0.47)	(0.52)	(0.59)	(0.30)	$ 0.30		(0.14)
Diluted net income (loss) per share (in dollars per share)	[1]	$ 0.21	[2]	$ 0.06	[3]	$ (0.47)	$ (0.52)	$ (0.59)	$ (0.30)	$ 0.29	[4]	$ (0.14)
1 Percent Convertible Senior Notes [Member]
Convertible Senior Notes [Abstract]
Interest rate on convertible senior notes		1.00%		1.00%				1.00%		1.00%			1.00%	1.00%	1.00%
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%	2.75%	2.75%
Stock Options [Member]
Effect of Diluted Securities [Abstract]
Shares issuable related to stock-based compensation (in shares)		2,189		2,129						3,974
Restricted Stock Awards [Member]
Effect of Diluted Securities [Abstract]
Shares issuable related to stock-based compensation (in shares)		403		202						376

[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 December, 2016, 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, 2016, the calculation excludes the 1 percent and 2 3/4 percent notes because the effect on diluted earnings per share would be anti-dilutive.

[3] For the three months ended September 30, 2016, 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 September 30, 2016, the calculation excludes the 1 percent and 2 3/4 percent notes because the effect on diluted earnings per share would be anti-dilutive.

[4] 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 was anti-dilutive.