Investments - Investments in Marketable Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 306,117	$ 261,767
Unrealized Gains	20	5
Unrealized Losses	(236)	(308)
Fair Value	$ 305,901	261,464
Investments, weighted average maturity	7 months
Investments in unrealized loss for more than 12 months | security	0
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 48,136	61,986
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	48,136	61,986
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	60,725	56,205
Unrealized Gains	1	3
Unrealized Losses	(50)	(99)
Fair Value	60,676	56,109
Agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	28,954	85,572
Unrealized Gains	2	0
Unrealized Losses	(26)	(111)
Fair Value	28,930	85,461
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	157,829	58,004
Unrealized Gains	17	2
Unrealized Losses	(160)	(98)
Fair Value	157,686	$ 57,908
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	10,473
Unrealized Gains	0
Unrealized Losses	0
Fair Value	$ 10,473