Fair Value of Financial Instruments (Contractual Maturity) (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Mature within One Year	$ 214,731
After One Year through Two Years	85,617
Over Two Years	33,890
Fair Value	334,238
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Mature within One Year	5,638
After One Year through Two Years	0
Over Two Years	0
Fair Value	5,638
U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Mature within One Year	173,546
After One Year through Two Years	46,816
Over Two Years
Fair Value	220,362
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Mature within One Year	33,098
After One Year through Two Years	23,251
Over Two Years	9,247
Fair Value	65,596
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Mature within One Year	2,449
After One Year through Two Years	15,550
Over Two Years	24,643
Fair Value	$ 42,642