Fair Value (Available-For-Sale Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Fair Value Disclosure [Line Items]
Amortized Cost	$ 168.8	$ 166.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(13.2)	(14.2)
Allowance for Credit Loss	(0.7)	(0.9)
Fair Value	154.9	151.6
U.S. Treasury debt securities
Fair Value Disclosure [Line Items]
Amortized Cost	70.1	67.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(4.2)	(4.5)
Allowance for Credit Loss	0.0	0.0
Fair Value	65.9	63.2
Debt Securities, Available-for-sale, Unrealized Loss Position	59.9	61.0
Corporate/Other debt securities
Fair Value Disclosure [Line Items]
Amortized Cost	98.7	99.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(9.0)	(9.7)
Allowance for Credit Loss	(0.7)	(0.9)
Fair Value	89.0	88.4
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 83.7	$ 85.5