Financial Instruments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) position	Dec. 31, 2015 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 130,860	$ 201,586
Realized gain	$ 200	0
Number of investments, unrealized loss position | position	27
Interest income	$ 1,500
Amortized Cost
Less than one year	48,265
Greater than one year but less than five years	83,320
Total	131,585
Fair Value
Less than one year	48,212
Greater than one year but less than five years	82,648
Total	130,860
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	142,362	208,968
Unrealized Gain		3
Unrealized Loss	(725)	(1,419)
Estimated Fair Value	141,637	207,552
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	10,777	5,966
Estimated Fair Value	10,777	5,966
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	57,846	104,537
Unrealized Gain		1
Unrealized Loss	(404)	(725)
Estimated Fair Value	57,442	103,813
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	56,261	79,491
Unrealized Loss	(147)	(638)
Estimated Fair Value	56,114	78,853
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	17,478	18,974
Unrealized Gain		2
Unrealized Loss	(174)	(56)
Estimated Fair Value	17,304	18,920
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	10,777	5,966
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 130,860	$ 201,586