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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,413.5	$ 9,169.6
Debt Securities, Gross unrealized gains	9.9	9.5
Debt Securities, Gross unrealized losses	(968.9)	(1,009.5)
Debt securities, Estimated fair value	7,454.5	8,169.6
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	130.9	308.5
Debt Securities, Gross unrealized gains	0.0	1.4
Debt Securities, Gross unrealized losses	(6.5)	(7.1)
Debt securities, Estimated fair value	124.4	302.8
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,399.1	1,670.6
Debt Securities, Gross unrealized gains	4.5	3.2
Debt Securities, Gross unrealized losses	(163.6)	(195.1)
Debt securities, Estimated fair value	1,240.0	1,478.7
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	229.5	208.0
Debt Securities, Gross unrealized gains	0.3	0.1
Debt Securities, Gross unrealized losses	(13.9)	(14.4)
Debt securities, Estimated fair value	215.9	193.7
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	242.3	247.9
Debt Securities, Gross unrealized gains	0.0	0.0
Debt Securities, Gross unrealized losses	(17.8)	(19.3)
Debt securities, Estimated fair value	224.5	228.6
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,968.1	5,253.4
Debt Securities, Gross unrealized gains	1.4	1.7
Debt Securities, Gross unrealized losses	(658.1)	(652.7)
Debt securities, Estimated fair value	4,311.4	4,602.4
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	972.7	1,004.4
Debt Securities, Gross unrealized gains	1.9	1.5
Debt Securities, Gross unrealized losses	(76.9)	(84.5)
Debt securities, Estimated fair value	897.7	921.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	470.9	476.8
Debt Securities, Gross unrealized gains	1.8	1.6
Debt Securities, Gross unrealized losses	(32.1)	(36.4)
Debt securities, Estimated fair value	$ 440.6	$ 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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