Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,989.9	$ 6,175.0
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	51.2
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	619.7
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	91.8
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	141.3
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	3,537.1
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	364.2
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	184.6
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,735.2
Percentage of debt securities in unrealized loss position	94.90%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 203.7
Percentage of debt securities in unrealized loss position	4.10%
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	$ 51.2
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	$ 604.8
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	$ 14.6
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	$ 89.6
Percentage of debt securities in unrealized loss position	97.60%
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	$ 1.5
Percentage of debt securities in unrealized loss position	1.60%
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	$ 141.3
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	$ 3,537.1
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	$ 196.4
Percentage of debt securities in unrealized loss position	53.90%
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	$ 131.0
Percentage of debt securities in unrealized loss position	36.00%
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	$ 114.8
Percentage of debt securities in unrealized loss position	62.10%
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	$ 56.6
Percentage of debt securities in unrealized loss position	30.70%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 51.0
Percentage of debt securities in unrealized loss position	1.00%
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	0.80%
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	$ 36.8
Percentage of debt securities in unrealized loss position	10.10%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 13.2
Percentage of debt securities in unrealized loss position	7.20%