Investments (Details)	3 Months Ended
	Mar. 31, 2017 USD ($) investments	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Summary of Investments
Available-for-sale securities, Amortized Cost	$ 305,764,000		$ 305,148,000
Available-for-sale securities, Gross Unrealized Gains	48,000		35,000
Available-for-sale securities, Gross Unrealized Losses	(271,000)		(402,000)
Available-for-sale securities, Estimated Fair Value	305,541,000		304,781,000
Other than temporary impairment losses	$ 0	$ 0
Number of investments that have been in an unrealized loss position for more than one year | investments	0
Corporate debt securities
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	$ 137,390,000		106,430,000
Available-for-sale securities, due in one to three years, Amortized Cost	111,161,000		139,742,000
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	8,000		3,000
Available-for-sale securities, due in one to three years, Gross Unrealized Gains	36,000		32,000
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	(86,000)		(69,000)
Available-for-sale securities, due in one to three years, Gross Unrealized Losses	(135,000)		(281,000)
Available-for-sale securities, due in one year or less, Estimated Fair Value	137,312,000		106,364,000
Available-for-sale securities, due in one to three years, Estimated Fair Value	111,062,000		139,493,000
U.S. treasury and government agency securities
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	2,513,000		1,021,000
Available-for-sale securities, due in one to three years, Amortized Cost	14,373,000		11,395,000
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	0		0
Available-for-sale securities, due in one to three years, Gross Unrealized Gains	4,000		0
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	(1,000)		0
Available-for-sale securities, due in one to three years, Gross Unrealized Losses	(49,000)		(52,000)
Available-for-sale securities, due in one year or less, Estimated Fair Value	2,512,000		1,021,000
Available-for-sale securities, due in one to three years, Estimated Fair Value	14,328,000		11,343,000
Commercial paper
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	26,875,000		40,560,000
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	0		0
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	0		0
Available-for-sale securities, due in one year or less, Estimated Fair Value	26,875,000		40,560,000
Certificates of deposit
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	12,000,000		6,000,000
Available-for-sale securities, due in one to three years, Amortized Cost	1,452,000
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	0		0
Available-for-sale securities, due in one to three years, Gross Unrealized Gains	0
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	0		0
Available-for-sale securities, due in one to three years, Gross Unrealized Losses	0
Available-for-sale securities, due in one year or less, Estimated Fair Value	12,000,000		$ 6,000,000
Available-for-sale securities, due in one to three years, Estimated Fair Value	$ 1,452,000