Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) position	Dec. 31, 2015 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 196,597	$ 201,586
Realized gain	$ 100
Number of investments, unrealized loss position | position	34
Interest income	$ 400
Amortized Cost
Less than one year	59,489
Greater than one year but less than five years	144,976
Total	204,465
Fair Value
Less than one year	59,464
Greater than one year but less than five years	145,396
Total	204,860
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	204,465	208,968
Unrealized Gain	479	3
Unrealized Loss	(84)	(1,419)
Estimated Fair Value	204,860	207,552
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	8,263	5,966
Estimated Fair Value	8,263	5,966
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	102,950	104,537
Unrealized Gain	183	1
Unrealized Loss	(37)	(725)
Estimated Fair Value	103,096	103,813
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	74,428	79,491
Unrealized Gain	228
Unrealized Loss	(40)	(638)
Estimated Fair Value	74,616	78,853
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	18,824	18,974
Unrealized Gain	68	2
Unrealized Loss	(7)	(56)
Estimated Fair Value	18,885	18,920
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	8,263	5,966
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 196,597	$ 201,586