Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 9,421.6	$ 3,883.2
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	247.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	1,005.1
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	183.1
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	444.0
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	6,047.5
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	1,093.9
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	400.7
Investment Grade \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 8,852.8
Percentage of debt securities in unrealized loss position	93.90%
Investment Grade \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 449.4
Percentage of debt securities in unrealized loss position	4.80%
Investment Grade \| U.S. Treasury bonds \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 247.3
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade \| U.S. Treasury bonds \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.0
Percentage of debt securities in unrealized loss position	0.00%
Investment Grade \| Municipal bonds \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 991.5
Percentage of debt securities in unrealized loss position	98.70%
Investment Grade \| Municipal bonds \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 13.3
Percentage of debt securities in unrealized loss position	1.30%
Investment Grade \| Foreign government bonds \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 174.9
Percentage of debt securities in unrealized loss position	95.50%
Investment Grade \| Foreign government bonds \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 7.5
Percentage of debt securities in unrealized loss position	4.10%
Investment Grade \| Governmental agency bonds \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 444.0
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade \| Governmental agency bonds \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.0
Percentage of debt securities in unrealized loss position	0.00%
Investment Grade \| Governmental agency mortgage-backed securities \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,047.5
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade \| Governmental agency mortgage-backed securities \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.0
Percentage of debt securities in unrealized loss position	0.00%
Investment Grade \| U.S. corporate debt securities \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 720.3
Percentage of debt securities in unrealized loss position	65.80%
Investment Grade \| U.S. corporate debt securities \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 273.0
Percentage of debt securities in unrealized loss position	25.00%
Investment Grade \| Foreign corporate debt securities \| A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 227.3
Percentage of debt securities in unrealized loss position	56.70%
Investment Grade \| Foreign corporate debt securities \| BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 155.6
Percentage of debt securities in unrealized loss position	38.90%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 119.4
Percentage of debt securities in unrealized loss position	1.30%
Non-Investment Grade \| U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.0
Percentage of debt securities in unrealized loss position	0.00%
Non-Investment Grade \| Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.3
Percentage of debt securities in unrealized loss position	0.00%
Non-Investment Grade \| Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.7
Percentage of debt securities in unrealized loss position	0.40%
Non-Investment Grade \| Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.0
Percentage of debt securities in unrealized loss position	0.00%
Non-Investment Grade \| Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0.0
Percentage of debt securities in unrealized loss position	0.00%
Non-Investment Grade \| U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 100.6
Percentage of debt securities in unrealized loss position	9.20%
Non-Investment Grade \| Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 17.8
Percentage of debt securities in unrealized loss position	4.40%