Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) position
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 201,586
Interest income	$ 300
Number of investments, unrealized loss position | position	73
Amortized Cost
Less than one year	$ 55,128
Greater than one year but less than five years	153,840
Total	208,968
Fair Value
Less than one year	55,010
Greater than one year but less than five years	152,542
Total	207,552
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	208,968
Unrealized Gain	3
Unrealized Loss	(1,419)
Estimated Fair Value	207,552
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	5,966
Estimated Fair Value	5,966
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	104,537
Unrealized Gain	1
Unrealized Loss	(725)
Estimated Fair Value	103,813
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	79,491
Unrealized Loss	(638)
Estimated Fair Value	78,853
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	18,974
Unrealized Gain	2
Unrealized Loss	(56)
Estimated Fair Value	18,920
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	5,966
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 201,586