Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,875.7	$ 7,645.8
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	95.6
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	1,094.4
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	142.1
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	230.3
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	4,235.0
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	724.8
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	353.5
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,278.5
Percentage of debt securities in unrealized loss position	91.30%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 464.0
Percentage of debt securities in unrealized loss position	6.70%
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	$ 95.6
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	$ 1,056.8
Percentage of debt securities in unrealized loss position	96.60%
Investment Grade | Municipal bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 37.3
Percentage of debt securities in unrealized loss position	3.40%
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	$ 140.1
Percentage of debt securities in unrealized loss position	98.60%
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	$ 1.3
Percentage of debt securities in unrealized loss position	0.90%
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	$ 230.3
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	$ 4,235.0
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	$ 356.2
Percentage of debt securities in unrealized loss position	49.10%
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	$ 265.4
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	$ 164.5
Percentage of debt securities in unrealized loss position	46.50%
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	$ 160.0
Percentage of debt securities in unrealized loss position	45.30%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 133.2
Percentage of debt securities in unrealized loss position	2.00%
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	0.50%
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	$ 103.2
Percentage of debt securities in unrealized loss position	14.30%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 29.0
Percentage of debt securities in unrealized loss position	8.20%