Preferred Stock and Stockholders' Equity (Summary of Options Outstanding and Exercisable) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number of Shares	20,454	17,663	16,195	14,704
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	6 years 9 months 14 days
Options Outstanding, Weighted Average Exercise Price	$ 5.22	$ 3.63	$ 2.98	$ 2.62
Options Outstanding, Aggregate Intrinsic Value	$ 263,129
Options Exercisable, Number of Shares	12,046
Options Exercisable, Weighted Average Exercise Price	$ 3.61
Options Exercisable, Aggregate Intrinsic Value	$ 174,019
$0.96 - $ 4.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Of Options, minimum	$ 0.96
Exercise Price Of Options, Maximum	$ 4.00
Options Outstanding, Number of Shares	10,548
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	5 years 8 months 12 days
Options Outstanding, Weighted Average Exercise Price	$ 2.82
Options Outstanding, Aggregate Intrinsic Value	$ 160,714
Options Exercisable, Number of Shares	8,399
Options Exercisable, Weighted Average Exercise Price	$ 2.62
Options Exercisable, Aggregate Intrinsic Value	$ 129,668
$4.01 - $ 9.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Of Options, minimum	$ 4.01
Exercise Price Of Options, Maximum	$ 9.00
Options Outstanding, Number of Shares	8,220
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	7 years 8 months 12 days
Options Outstanding, Weighted Average Exercise Price	$ 6.57
Options Outstanding, Aggregate Intrinsic Value	$ 94,476
Options Exercisable, Number of Shares	3,503
Options Exercisable, Weighted Average Exercise Price	$ 5.64
Options Exercisable, Aggregate Intrinsic Value	$ 43,521
$9.01 - $ 17.00
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Of Options, minimum	$ 9.01
Exercise Price Of Options, Maximum	$ 17.00
Options Outstanding, Number of Shares	1,296
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	9 years 4 months 24 days
Options Outstanding, Weighted Average Exercise Price	$ 11.94
Options Outstanding, Aggregate Intrinsic Value	$ 7,939
Options Exercisable, Number of Shares	140
Options Exercisable, Weighted Average Exercise Price	$ 12.13
Options Exercisable, Aggregate Intrinsic Value	$ 830
$17.01 - $ 19.73
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price Of Options, minimum	$ 17.01
Exercise Price Of Options, Maximum	$ 19.73
Options Outstanding, Number of Shares	390
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	9 years 9 months 18 days
Options Outstanding, Weighted Average Exercise Price	$ 19.23
Options Exercisable, Number of Shares	4
Options Exercisable, Weighted Average Exercise Price	$ 19.73