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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,895.2	$ 9,169.6
Debt Securities, Gross unrealized gains	29.6	9.5
Debt Securities, Gross unrealized losses	(767.3)	(1,009.5)
Debt securities, Estimated fair value	7,157.5	8,169.6
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	203.3	308.5
Debt Securities, Gross unrealized gains	0.5	1.4
Debt Securities, Gross unrealized losses	(4.5)	(7.1)
Debt securities, Estimated fair value	199.3	302.8
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,373.7	1,670.6
Debt Securities, Gross unrealized gains	8.8	3.2
Debt Securities, Gross unrealized losses	(136.7)	(195.1)
Debt securities, Estimated fair value	1,245.8	1,478.7
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	228.4	208.0
Debt Securities, Gross unrealized gains	1.4	0.1
Debt Securities, Gross unrealized losses	(10.5)	(14.4)
Debt securities, Estimated fair value	219.3	193.7
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	207.7	247.9
Debt Securities, Gross unrealized gains	0.2	0.0
Debt Securities, Gross unrealized losses	(12.5)	(19.3)
Debt securities, Estimated fair value	195.4	228.6
Collateralized Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,396.2	5,253.4
Debt Securities, Gross unrealized gains	6.3	1.7
Debt Securities, Gross unrealized losses	(526.8)	(652.7)
Debt securities, Estimated fair value	3,875.7	4,602.4
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,007.0	1,004.4
Debt Securities, Gross unrealized gains	6.6	1.5
Debt Securities, Gross unrealized losses	(55.2)	(84.5)
Debt securities, Estimated fair value	958.4	921.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	478.9	476.8
Debt Securities, Gross unrealized gains	5.8	1.6
Debt Securities, Gross unrealized losses	(21.1)	(36.4)
Debt securities, Estimated fair value	$ 463.6	$ 442.0

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- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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