Investments - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total available-for-sale securities
Amortized Cost	$ 66,807	$ 116,064
Gross Unrealized Gains		10
Gross Unrealized Losses	(20)	(24)
Estimated Fair Value	66,787	116,050
Available-for-sale securities in a loss position
Estimated Fair Value	43,788	68,275
Gross Unrealized Losses	(20)	(24)
Short-term investments that had been in a continuous loss position for more than 12 months	0	0
Realized gains or losses	0
Other net
Available-for-sale securities in a loss position
Realized gains or losses		0	$ 0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	40,013	94,935
Gross Unrealized Gains		6
Gross Unrealized Losses	(5)	(23)
Estimated Fair Value	40,008	94,918
Available-for-sale securities in a loss position
Estimated Fair Value	20,002	64,922
Gross Unrealized Losses	(5)	(23)
Government agency securities
Total available-for-sale securities
Amortized Cost	10,020	12,985
Gross Unrealized Gains		3
Gross Unrealized Losses	(8)
Estimated Fair Value	10,012	12,988
Available-for-sale securities in a loss position
Estimated Fair Value	10,012
Gross Unrealized Losses	(8)
Corporate debt
Total available-for-sale securities
Amortized Cost	13,780	8,144
Gross Unrealized Gains		1
Gross Unrealized Losses	(7)	(1)
Estimated Fair Value	13,773	8,144
Available-for-sale securities in a loss position
Estimated Fair Value	13,774	3,353
Gross Unrealized Losses	(7)	$ (1)
Commercial paper
Total available-for-sale securities
Amortized Cost	2,994
Estimated Fair Value	$ 2,994