Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 8,018.9	$ 7,265.9
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,271.0
Percentage of debt securities by credit rating agencies	90.60%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 533.4
Percentage of debt securities by credit rating agencies	6.70%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 214.5
Percentage of debt securities by credit rating agencies	2.70%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 170.2	175.6
U.S. Treasury bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 170.2
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	$ 876.7	776.5
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 856.7
Percentage of debt securities by credit rating agencies	97.80%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 19.7
Percentage of debt securities by credit rating agencies	2.20%
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	$ 225.5	211.6
Foreign government bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 220.3
Percentage of debt securities by credit rating agencies	97.70%
Foreign government bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4.5
Percentage of debt securities by credit rating agencies	2.00%
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	$ 206.3	189.8
Governmental agency bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 206.3
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	$ 5,037.9	4,502.3
Governmental agency mortgage-backed securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 5,037.9
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	$ 1,010.0	925.6
U.S. corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 526.1
Percentage of debt securities by credit rating agencies	52.00%
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 312.6
Percentage of debt securities by credit rating agencies	31.00%
U.S. corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 171.3
Percentage of debt securities by credit rating agencies	17.00%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 492.3	$ 484.5
Foreign corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 253.5
Percentage of debt securities by credit rating agencies	51.50%
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 196.6
Percentage of debt securities by credit rating agencies	39.90%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 42.2
Percentage of debt securities by credit rating agencies	8.60%