Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,334.2	$ 7,730.9
Debt securities, Estimated fair value	8,018.9	7,265.9
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	33.1
Amortized cost, Due after one through five years	107.8
Amortized cost, Due after five through ten years	5.4
Amortized cost, Due after ten years	24.9
Debt Securities, Amortized cost	171.2	180.7
Estimated fair value, Due in one year or less	32.9
Estimated fair value, Due after one through five years	107.4
Estimated fair value, Due after five through ten years	5.3
Estimated fair value, Due after ten years	24.6
Debt securities, Estimated fair value	170.2	175.6
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	7.2
Amortized cost, Due after one through five years	94.3
Amortized cost, Due after five through ten years	359.3
Amortized cost, Due after ten years	485.8
Debt Securities, Amortized cost	946.6	844.9
Estimated fair value, Due in one year or less	7.2
Estimated fair value, Due after one through five years	91.9
Estimated fair value, Due after five through ten years	331.0
Estimated fair value, Due after ten years	446.6
Debt securities, Estimated fair value	876.7	776.5
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	16.4
Amortized cost, Due after one through five years	164.4
Amortized cost, Due after five through ten years	45.3
Amortized cost, Due after ten years	3.7
Debt Securities, Amortized cost	229.8	217.1
Estimated fair value, Due in one year or less	16.4
Estimated fair value, Due after one through five years	163.1
Estimated fair value, Due after five through ten years	43.3
Estimated fair value, Due after ten years	2.7
Debt securities, Estimated fair value	225.5	211.6
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	1.2
Amortized cost, Due after one through five years	1.4
Amortized cost, Due after five through ten years	168.8
Amortized cost, Due after ten years	42.7
Debt Securities, Amortized cost	214.1	203.8
Estimated fair value, Due in one year or less	1.2
Estimated fair value, Due after one through five years	1.4
Estimated fair value, Due after five through ten years	168.6
Estimated fair value, Due after ten years	35.1
Debt securities, Estimated fair value	206.3	189.8
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	12.0
Amortized cost, Due after one through five years	558.2
Amortized cost, Due after five through ten years	354.5
Amortized cost, Due after ten years	91.5
Debt Securities, Amortized cost	1,016.2	948.4
Estimated fair value, Due in one year or less	11.9
Estimated fair value, Due after one through five years	560.8
Estimated fair value, Due after five through ten years	356.0
Estimated fair value, Due after ten years	81.3
Debt securities, Estimated fair value	1,010.0	925.6
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	19.4
Amortized cost, Due after one through five years	318.4
Amortized cost, Due after five through ten years	123.3
Amortized cost, Due after ten years	29.8
Debt Securities, Amortized cost	490.9	491.6
Estimated fair value, Due in one year or less	19.4
Estimated fair value, Due after one through five years	321.4
Estimated fair value, Due after five through ten years	124.3
Estimated fair value, Due after ten years	27.2
Debt securities, Estimated fair value	492.3	$ 484.5
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	89.3
Amortized cost, Due after one through five years	1,244.5
Amortized cost, Due after five through ten years	1,056.6
Amortized cost, Due after ten years	678.4
Debt Securities, Amortized cost	3,068.8
Estimated fair value, Due in one year or less	89.0
Estimated fair value, Due after one through five years	1,246.0
Estimated fair value, Due after five through ten years	1,028.5
Estimated fair value, Due after ten years	617.5
Debt securities, Estimated fair value	2,981.0
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	5,265.4
Estimated fair value	$ 5,037.9