Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,265.9	$ 7,157.5
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 6,532.6
Percentage of debt securities by credit rating agencies	89.90%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 521.0
Percentage of debt securities by credit rating agencies	7.20%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 212.3
Percentage of debt securities by credit rating agencies	2.90%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 175.6	199.3
U.S. Treasury bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 175.6
Percentage of debt securities by credit rating agencies	100.00%
U.S. Treasury bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.0
Percentage of debt securities by credit rating agencies	0.00%
U.S. Treasury bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.0
Percentage of debt securities by credit rating agencies	0.00%
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 776.5	1,245.8
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 755.1
Percentage of debt securities by credit rating agencies	97.30%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 21.1
Percentage of debt securities by credit rating agencies	2.70%
Municipal bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.3
Percentage of debt securities by credit rating agencies	0.00%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 211.6	219.3
Foreign government bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 206.5
Percentage of debt securities by credit rating agencies	97.60%
Foreign government bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4.4
Percentage of debt securities by credit rating agencies	2.10%
Foreign government bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.7
Percentage of debt securities by credit rating agencies	0.30%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 189.8	195.4
Governmental agency bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 189.8
Percentage of debt securities by credit rating agencies	100.00%
Governmental agency bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.0
Percentage of debt securities by credit rating agencies	0.00%
Governmental agency bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.0
Percentage of debt securities by credit rating agencies	0.00%
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4,502.3	3,875.7
Governmental agency mortgage-backed securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4,502.3
Percentage of debt securities by credit rating agencies	100.00%
Governmental agency mortgage-backed securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.0
Percentage of debt securities by credit rating agencies	0.00%
Governmental agency mortgage-backed securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.0
Percentage of debt securities by credit rating agencies	0.00%
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 925.6	958.4
U.S. corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 444.1
Percentage of debt securities by credit rating agencies	48.00%
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 314.5
Percentage of debt securities by credit rating agencies	34.00%
U.S. corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 167.0
Percentage of debt securities by credit rating agencies	18.00%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 484.5	$ 463.6
Foreign corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 259.2
Percentage of debt securities by credit rating agencies	53.50%
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 181.0
Percentage of debt securities by credit rating agencies	37.40%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 44.3
Percentage of debt securities by credit rating agencies	9.10%