Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,730.9	$ 7,895.2
Debt securities, Estimated fair value	7,265.9	7,157.5
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	32.9
Amortized cost, Due after one through five years	117.6
Amortized cost, Due after five through ten years	4.7
Amortized cost, Due after ten years	25.5
Debt Securities, Amortized cost	180.7	203.3
Estimated fair value, Due in one year or less	32.8
Estimated fair value, Due after one through five years	114.7
Estimated fair value, Due after five through ten years	4.6
Estimated fair value, Due after ten years	23.5
Debt securities, Estimated fair value	175.6	199.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	7.1
Amortized cost, Due after one through five years	83.0
Amortized cost, Due after five through ten years	283.1
Amortized cost, Due after ten years	471.7
Debt Securities, Amortized cost	844.9	1,373.7
Estimated fair value, Due in one year or less	7.1
Estimated fair value, Due after one through five years	80.5
Estimated fair value, Due after five through ten years	247.6
Estimated fair value, Due after ten years	441.3
Debt securities, Estimated fair value	776.5	1,245.8
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	13.0
Amortized cost, Due after one through five years	123.7
Amortized cost, Due after five through ten years	73.7
Amortized cost, Due after ten years	6.7
Debt Securities, Amortized cost	217.1	228.4
Estimated fair value, Due in one year or less	13.0
Estimated fair value, Due after one through five years	125.1
Estimated fair value, Due after five through ten years	67.9
Estimated fair value, Due after ten years	5.6
Debt securities, Estimated fair value	211.6	219.3
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	8.6
Amortized cost, Due after one through five years	1.4
Amortized cost, Due after five through ten years	149.5
Amortized cost, Due after ten years	44.3
Debt Securities, Amortized cost	203.8	207.7
Estimated fair value, Due in one year or less	8.6
Estimated fair value, Due after one through five years	1.4
Estimated fair value, Due after five through ten years	144.2
Estimated fair value, Due after ten years	35.6
Debt securities, Estimated fair value	189.8	195.4
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	12.4
Amortized cost, Due after one through five years	545.2
Amortized cost, Due after five through ten years	295.6
Amortized cost, Due after ten years	95.2
Debt Securities, Amortized cost	948.4	1,007.0
Estimated fair value, Due in one year or less	12.5
Estimated fair value, Due after one through five years	539.0
Estimated fair value, Due after five through ten years	290.5
Estimated fair value, Due after ten years	83.6
Debt securities, Estimated fair value	925.6	958.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	22.1
Amortized cost, Due after one through five years	309.9
Amortized cost, Due after five through ten years	128.4
Amortized cost, Due after ten years	31.2
Debt Securities, Amortized cost	491.6	478.9
Estimated fair value, Due in one year or less	22.0
Estimated fair value, Due after one through five years	308.4
Estimated fair value, Due after five through ten years	126.1
Estimated fair value, Due after ten years	28.0
Debt securities, Estimated fair value	484.5	$ 463.6
Aggregate Debt Securities Excluding Mortgage Backed And Asset Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	96.1
Amortized cost, Due after one through five years	1,180.8
Amortized cost, Due after five through ten years	935.0
Amortized cost, Due after ten years	674.6
Debt Securities, Amortized cost	2,886.5
Estimated fair value, Due in one year or less	96.0
Estimated fair value, Due after one through five years	1,169.1
Estimated fair value, Due after five through ten years	880.9
Estimated fair value, Due after ten years	617.6
Debt securities, Estimated fair value	2,763.6
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	4,844.4
Estimated fair value	$ 4,502.3