Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Accruals and reserves	$ 27,022	$ 30,317
Deferred revenue	62,085	65,016
Tax credits	81,720	80,772
Net operating losses	54,747	36,674
Stock based compensation	30,936	21,714
Depreciation and amortization	0	4,939
Valuation allowance	(85,400)	(76,789)
Total deferred tax assets	171,110	162,643
Deferred tax liabilities:
Acquired technology	(15,681)	(2,882)
Depreciation and amortization	(5,044)	0
Prepaid expenses	(23,213)	(7,414)
Other	(5,249)	(222)
Total deferred tax liabilities	(49,187)	(10,518)
Total net deferred tax assets	$ 121,923	$ 152,125