Fair Value of Financial Instruments (Contractual Maturity) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	$ 266,734
After One Year through Two Years	56,615
Over Two Years	1,501
Fair Value	324,850
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	3,237
After One Year through Two Years	0
Over Two Years	0
Fair Value	3,237
U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	203,649
After One Year through Two Years	18,751
Over Two Years	0
Fair Value	222,400
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	30,822
After One Year through Two Years	33,531
Over Two Years	1,501
Fair Value	65,854
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	29,026
After One Year through Two Years	4,333
Over Two Years	0
Fair Value	$ 33,359