Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,859.0	$ 7,895.2
Debt Securities, Gross unrealized gains	17.9	29.6
Debt Securities, Gross unrealized losses	(822.5)	(767.3)
Debt securities, Estimated fair value	7,054.4	7,157.5
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	163.1	203.3
Debt Securities, Gross unrealized gains	0.0	0.5
Debt Securities, Gross unrealized losses	(5.9)	(4.5)
Debt securities, Estimated fair value	157.2	199.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,381.2	1,373.7
Debt Securities, Gross unrealized gains	7.1	8.8
Debt Securities, Gross unrealized losses	(139.0)	(136.7)
Debt securities, Estimated fair value	1,249.3	1,245.8
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	209.9	228.4
Debt Securities, Gross unrealized gains	0.2	1.4
Debt Securities, Gross unrealized losses	(10.7)	(10.5)
Debt securities, Estimated fair value	199.4	219.3
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	214.3	207.7
Debt Securities, Gross unrealized gains	0.0	0.2
Debt Securities, Gross unrealized losses	(13.9)	(12.5)
Debt securities, Estimated fair value	200.4	195.4
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,316.7	4,396.2
Debt Securities, Gross unrealized gains	2.2	6.3
Debt Securities, Gross unrealized losses	(573.0)	(526.8)
Debt securities, Estimated fair value	3,745.9	3,875.7
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,075.4	1,007.0
Debt Securities, Gross unrealized gains	4.4	6.6
Debt Securities, Gross unrealized losses	(58.6)	(55.2)
Debt securities, Estimated fair value	1,021.2	958.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	498.4	478.9
Debt Securities, Gross unrealized gains	4.0	5.8
Debt Securities, Gross unrealized losses	(21.4)	(21.1)
Debt securities, Estimated fair value	$ 481.0	$ 463.6

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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