Fair Value of Financial Instruments (Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	$ 177,473	$ 113,614
After One Year through Two Years	38,962	39,699
Over Two Years	0	3,578
Fair Value	216,435	156,891
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	28,378	27,794
After One Year through Two Years	0	0
Over Two Years	0	0
Fair Value	28,378	27,794
U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	127,619	73,385
After One Year through Two Years	29,430	32,541
Over Two Years	0	0
Fair Value	157,049	105,926
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	15,149	12,435
After One Year through Two Years	9,532	3,747
Over Two Years	0	0
Fair Value	24,681	16,182
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	6,327	0
After One Year through Two Years	0	3,411
Over Two Years	0	3,578
Fair Value	$ 6,327	$ 6,989