Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,763.2	$ 3,865.7
Unrealized losses, Less than 12 months	(69.9)	(268.4)
Estimated fair value, 12 months or longer	5,112.5	3,780.1
Unrealized losses, 12 months or longer	(799.9)	(741.1)
Estimated fair value, Total	6,875.7	7,645.8
Unrealized losses, Total	(869.8)	(1,009.5)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	40.1	108.0
Unrealized losses, Less than 12 months	(0.1)	(1.4)
Estimated fair value, 12 months or longer	55.5	49.5
Unrealized losses, 12 months or longer	(5.3)	(5.7)
Estimated fair value, Total	95.6	157.5
Unrealized losses, Total	(5.4)	(7.1)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	299.8	813.4
Unrealized losses, Less than 12 months	(5.9)	(55.8)
Estimated fair value, 12 months or longer	794.6	540.0
Unrealized losses, 12 months or longer	(147.9)	(139.3)
Estimated fair value, Total	1,094.4	1,353.4
Unrealized losses, Total	(153.8)	(195.1)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	45.3	142.1
Unrealized losses, Less than 12 months	(0.4)	(5.9)
Estimated fair value, 12 months or longer	96.8	45.7
Unrealized losses, 12 months or longer	(10.8)	(8.5)
Estimated fair value, Total	142.1	187.8
Unrealized losses, Total	(11.2)	(14.4)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	146.2	172.7
Unrealized losses, Less than 12 months	(3.3)	(8.2)
Estimated fair value, 12 months or longer	84.1	55.8
Unrealized losses, 12 months or longer	(13.0)	(11.1)
Estimated fair value, Total	230.3	228.5
Unrealized losses, Total	(16.3)	(19.3)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,004.1	1,859.6
Unrealized losses, Less than 12 months	(53.3)	(141.4)
Estimated fair value, 12 months or longer	3,230.9	2,626.8
Unrealized losses, 12 months or longer	(528.6)	(511.3)
Estimated fair value, Total	4,235.0	4,486.4
Unrealized losses, Total	(581.9)	(652.7)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	142.4	528.3
Unrealized losses, Less than 12 months	(4.3)	(38.2)
Estimated fair value, 12 months or longer	582.4	325.2
Unrealized losses, 12 months or longer	(66.7)	(46.3)
Estimated fair value, Total	724.8	853.5
Unrealized losses, Total	(71.0)	(84.5)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	85.3	241.6
Unrealized losses, Less than 12 months	(2.6)	(17.5)
Estimated fair value, 12 months or longer	268.2	137.1
Unrealized losses, 12 months or longer	(27.6)	(18.9)
Estimated fair value, Total	353.5	378.7
Unrealized losses, Total	$ (30.2)	$ (36.4)

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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