Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Total available-for-sale securities
Amortized Cost	$ 258,330		$ 66,807
Gross Unrealized Losses	(134)		(20)
Estimated Fair Value	258,196		66,787
Available-for-sale securities in a loss position
Estimated Fair Value	239,052		43,788
Gross Unrealized Losses	(134)		(20)
Investments that had been in a continuous loss position for more than 12 months	0		0
Amortized costs of contractual maturities of available-for-sale securities
Realized gains	0	$ 0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	180,101		40,013
Gross Unrealized Losses	(88)		(5)
Estimated Fair Value	180,013		40,008
Available-for-sale securities in a loss position
Estimated Fair Value	180,013		20,002
Gross Unrealized Losses	(88)		(5)
Government agency securities
Total available-for-sale securities
Amortized Cost	30,021		10,020
Gross Unrealized Losses	(23)		(8)
Estimated Fair Value	29,998		10,012
Available-for-sale securities in a loss position
Estimated Fair Value	29,998		10,012
Gross Unrealized Losses	(23)		(8)
Corporate debt
Total available-for-sale securities
Amortized Cost	30,265		13,780
Gross Unrealized Losses	(23)		(7)
Estimated Fair Value	30,242		13,773
Available-for-sale securities in a loss position
Estimated Fair Value	29,041		13,774
Gross Unrealized Losses	(23)		(7)
Commercial paper
Total available-for-sale securities
Amortized Cost	17,943		2,994
Estimated Fair Value	$ 17,943		$ 2,994