Investments (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) investments	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) investments	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Summary of Investments
Available-for-sale securities, amortized cost	$ 139,145,000		$ 139,145,000		$ 305,148,000
Available-for-sale securities, gross unrealized gains	69,000		69,000		35,000
Available-for-sale securities, gross unrealized losses	(119,000)		(119,000)		(402,000)
Available-for-sale securities, estimated fair value	139,095,000		139,095,000		304,781,000
Other than temporary impairment losses	$ 0	$ 0	$ 0	$ 0
Number of investments that have been in an unrealized loss position for more than one year | investments	0		0
Corporate debt securities
Summary of Investments
Available-for-sale securities, due in one year or less, amortized cost	$ 47,117,000		$ 47,117,000		106,430,000
Available-for-sale securities, due in one to three years, amortized cost	45,992,000		45,992,000		139,742,000
Available-for-sale securities, due in one year or less, gross unrealized gains	14,000		14,000		3,000
Available-for-sale securities, due in one to three years, gross unrealized gains	52,000		52,000		32,000
Available-for-sale securities, due in one year or less, gross unrealized losses	(25,000)		(25,000)		(69,000)
Available-for-sale securities, due in one to three years, gross unrealized losses	(48,000)		(48,000)		(281,000)
Available-for-sale securities, due in one year or less, estimated fair value	47,106,000		47,106,000		106,364,000
Available-for-sale securities, due in one to three years, estimated fair value	45,996,000		45,996,000		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	1,501,000		1,501,000		1,021,000
Available-for-sale securities, due in one to three years, amortized cost	9,375,000		9,375,000		11,395,000
Available-for-sale securities, due in one year or less, gross unrealized gains	0		0		0
Available-for-sale securities, due in one to three years, gross unrealized gains	3,000		3,000		0
Available-for-sale securities, due in one year or less, gross unrealized losses	0		0		0
Available-for-sale securities, due in one to three years, gross unrealized losses	(46,000)		(46,000)		(52,000)
Available-for-sale securities, due in one year or less, estimated fair value	1,501,000		1,501,000		1,021,000
Available-for-sale securities, due in one to three years, estimated fair value	9,332,000		9,332,000		11,343,000
Commercial paper
Summary of Investments
Available-for-sale securities, due in one year or less, amortized cost	16,708,000		16,708,000		40,560,000
Available-for-sale securities, due in one year or less, gross unrealized gains	0		0		0
Available-for-sale securities, due in one year or less, gross unrealized losses	0		0		0
Available-for-sale securities, due in one year or less, estimated fair value	16,708,000		16,708,000		40,560,000
Certificates of deposit
Summary of Investments
Available-for-sale securities, due in one year or less, amortized cost	17,000,000		17,000,000		6,000,000
Available-for-sale securities, due in one to three years, amortized cost	1,452,000		1,452,000
Available-for-sale securities, due in one year or less, gross unrealized gains	0		0		0
Available-for-sale securities, due in one to three years, gross unrealized gains	0		0
Available-for-sale securities, due in one year or less, gross unrealized losses	0		0		0
Available-for-sale securities, due in one to three years, gross unrealized losses	0		0
Available-for-sale securities, due in one year or less, estimated fair value	17,000,000		17,000,000		$ 6,000,000
Available-for-sale securities, due in one to three years, estimated fair value	$ 1,452,000		$ 1,452,000