Restricted Securities Available for Sale (Schedule of Restricted Securities Available for Sale by Aging Category) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Restricted Securities Available For Sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 50.0	$ 33.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1.6	0.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4.6	1.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.4	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	54.6	34.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	2.0	0.4
Corporate Bond Securities [Member]
Restricted Securities Available For Sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	21.2	15.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.7	0.2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4.6	1.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.4	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	25.8	16.3
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	1.1	0.2
US Government and Agency Securities [Member]
Restricted Securities Available For Sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	19.2	10.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.7	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	19.2	10.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0.7	0.1
Asset-backed Securities [Member]
Restricted Securities Available For Sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	8.3	8.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.2	0.1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	8.3	8.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	0.2	0.1
Commercial Paper [Member]
Restricted Securities Available For Sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	1.0
Mortgage-backed Securities [Member]
Restricted Securities Available For Sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0.3	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0.0	0.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 0.3	$ 0.0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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