Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,175.0	$ 5,879.6
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	134.7
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	665.3
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	102.5
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	182.3
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	4,267.6
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	552.7
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	269.9
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 5,813.5
Percentage of debt securities in unrealized loss position	94.10%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 312.5
Percentage of debt securities in unrealized loss position	5.10%
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	$ 134.7
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	$ 649.2
Percentage of debt securities in unrealized loss position	97.60%
Investment Grade | Municipal bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 15.8
Percentage of debt securities in unrealized loss position	2.40%
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	$ 97.4
Percentage of debt securities in unrealized loss position	95.00%
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	$ 4.4
Percentage of debt securities in unrealized loss position	4.30%
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	$ 182.3
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	$ 4,267.6
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	$ 317.2
Percentage of debt securities in unrealized loss position	57.40%
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	$ 202.4
Percentage of debt securities in unrealized loss position	36.60%
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	$ 165.1
Percentage of debt securities in unrealized loss position	61.20%
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	$ 89.9
Percentage of debt securities in unrealized loss position	33.30%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 49.0
Percentage of debt securities in unrealized loss position	0.80%
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	0.70%
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	$ 33.1
Percentage of debt securities in unrealized loss position	6.00%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 14.9
Percentage of debt securities in unrealized loss position	5.50%