Stockholders' Equity, Preferred and Common Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock [Abstract]
Common stock, shares authorized to issue (in shares)	300,000,000	300,000,000
Common stock, shares issued (in shares)	124,976,373	121,636,273
Common stock, shares outstanding (in shares)	124,976,373	121,636,273
Net proceeds from stock option exercises, vesting of restricted stock units, and ESPP purchases	$ 22,931	$ 13,417	$ 24,888
Preferred Stock [Member]
Preferred Stock [Abstract]
Preferred stock, shares authorized (in shares)	15,000,000
Preferred stock, shares outstanding (in shares)	0
Series C Junior Participating Preferred Stock [Member]
Preferred Stock [Abstract]
Preferred stock, shares issued (in shares)	0
Preferred stock, shares outstanding (in shares)	0
Common Stock [Member]
Common Stock [Abstract]
Common stock, shares authorized to issue (in shares)	300,000,000	300,000,000
Common stock, shares issued (in shares)	124,976,373	121,636,273
Common stock, shares outstanding (in shares)	124,973,373	121,636,273
Common shares reserved for future issuance (in shares)	18,419,727
Number of shares issued for stock option exercises, vesting of restricted stock units, and ESPP purchases (in shares)	1,706,000	1,285,000	1,908,000
Net proceeds from stock option exercises, vesting of restricted stock units, and ESPP purchases	$ 22,900	$ 13,700	$ 24,900