Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 198,647	$ 198,647	$ 201,586
Realized gain	100	100
Interest income	400	800
Amortized Cost
Less than one year	60,621	60,621
Greater than one year but less than five years	145,922	145,922
Total	206,543	206,543
Fair Value
Less than one year	60,623	60,623
Greater than one year but less than five years	146,862	146,862
Total	207,485	207,485
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	206,543	206,543	208,968
Unrealized Gain	951	951	3
Unrealized Loss	(9)	(9)	(1,419)
Estimated Fair Value	207,485	207,485	207,552
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	8,838	8,838	5,966
Estimated Fair Value	8,838	8,838	5,966
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	102,426	102,426	104,537
Unrealized Gain	394	394	1
Unrealized Loss			(725)
Estimated Fair Value	102,820	102,820	103,813
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	77,502	77,502	79,491
Unrealized Gain	410	410
Unrealized Loss	(9)	(9)	(638)
Estimated Fair Value	77,903	77,903	78,853
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	17,777	17,777	18,974
Unrealized Gain	147	147	2
Unrealized Loss			(56)
Estimated Fair Value	17,924	17,924	18,920
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	8,838	8,838	5,966
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 198,647	$ 198,647	$ 201,586