Marketable Securities - Unrealized losses (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Fair value
Fair value, less than 12 months	$ 126,636	$ 99,787
Fair value, 12 months or greater	214,492	238,684
Unrealized losses
Unrealized losses, Less than 12 months	(153)	(149)
Unrealized losses, 12 months or greater	(1,060)	(1,333)
Corporate Securities
Fair value
Fair value, less than 12 months	78,164	82,215
Fair value, 12 months or greater	72,512	88,990
Unrealized losses
Unrealized losses, Less than 12 months	(82)	(133)
Unrealized losses, 12 months or greater	(515)	(784)
Government Securities
Fair value
Fair value, less than 12 months	48,472	17,573
Fair value, 12 months or greater	141,980	149,694
Unrealized losses
Unrealized losses, Less than 12 months	(71)	(16)
Unrealized losses, 12 months or greater	$ (545)	$ (549)