Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,054.4	$ 7,157.5
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 6,264.3
Percentage of debt securities by credit rating agencies	88.80%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 603.0
Percentage of debt securities by credit rating agencies	8.50%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 187.1
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	$ 157.2	199.3
U.S. Treasury bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 157.2
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	$ 1,249.3	1,245.8
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1,223.0
Percentage of debt securities by credit rating agencies	97.90%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 24.6
Percentage of debt securities by credit rating agencies	2.00%
Municipal bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1.7
Percentage of debt securities by credit rating agencies	0.10%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 199.4	219.3
Foreign government bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 193.9
Percentage of debt securities by credit rating agencies	97.20%
Foreign government bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4.7
Percentage of debt securities by credit rating agencies	2.40%
Foreign government bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0.8
Percentage of debt securities by credit rating agencies	0.40%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 200.4	195.4
Governmental agency bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 200.4
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	$ 3,745.9	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	$ 3,745.9
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	$ 1,021.2	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	$ 492.0
Percentage of debt securities by credit rating agencies	48.10%
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 380.5
Percentage of debt securities by credit rating agencies	37.30%
U.S. corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 148.7
Percentage of debt securities by credit rating agencies	14.60%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 481.0	$ 463.6
Foreign corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 251.9
Percentage of debt securities by credit rating agencies	52.30%
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 193.2
Percentage of debt securities by credit rating agencies	40.20%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 35.9
Percentage of debt securities by credit rating agencies	7.50%