Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,454.5	$ 8,169.6
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 6,706.7
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	$ 571.9
Percentage of debt securities by credit rating agencies	7.70%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 175.9
Percentage of debt securities by credit rating agencies	2.40%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 124.4	302.8
U.S. Treasury bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 124.4
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	$ 1,240.0	1,478.7
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1,192.3
Percentage of debt securities by credit rating agencies	96.20%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 47.4
Percentage of debt securities by credit rating agencies	3.80%
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	$ 215.9	193.7
Foreign government bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 210.4
Percentage of debt securities by credit rating agencies	97.50%
Foreign government bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4.8
Percentage of debt securities by credit rating agencies	2.20%
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	$ 224.5	228.6
Governmental agency bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 224.5
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	$ 4,311.4	4,602.4
Governmental agency mortgage-backed securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4,311.4
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	$ 897.7	921.4
U.S. corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 433.5
Percentage of debt securities by credit rating agencies	48.30%
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 324.7
Percentage of debt securities by credit rating agencies	36.20%
U.S. corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 139.5
Percentage of debt securities by credit rating agencies	15.50%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 440.6	$ 442.0
Foreign corporate debt securities | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 210.2
Percentage of debt securities by credit rating agencies	47.70%
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 195.0
Percentage of debt securities by credit rating agencies	44.30%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 35.4
Percentage of debt securities by credit rating agencies	8.00%