Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Amortized Cost
Amortized cost	$ 171,491	$ 218,070
Gross Unrealized Gains
Gross Unrealized Gains	52	31
Gross Unrealized Losses
Gross Unrealized Losses	(38)	(320)
Fair Value
Fair Value	171,505	217,781
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	700	1,500
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	61,179	48,871
Maturing after one year through three years	334	15,940
Amortized cost	61,513	64,811
Gross Unrealized Gains
Maturing in one year or less	28	4
Maturing after one year through three years	12	24
Gross Unrealized Gains	40	28
Gross Unrealized Losses
Maturing in one year or less	(4)	(20)
Maturing after one year through three years		(57)
Gross Unrealized Losses	(4)	(77)
Fair Value
Maturing in one year or less	61,203	48,855
Maturing after one year through three years	346	15,907
Fair Value	61,549	64,762
Corporate debt securities
Amortized Cost
Maturing in one year or less	109,978	129,327
Maturing after one year through three years		23,932
Amortized cost	109,978	153,259
Gross Unrealized Gains
Maturing in one year or less	12	2
Maturing after one year through three years		1
Gross Unrealized Gains	12	3
Gross Unrealized Losses
Maturing in one year or less	(34)	(141)
Maturing after one year through three years		(102)
Gross Unrealized Losses	(34)	(243)
Fair Value
Maturing in one year or less	109,956	129,188
Maturing after one year through three years		23,831
Fair Value	$ 109,956	$ 153,019