Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,892.7	$ 7,895.2
Debt securities, Estimated fair value	7,080.7	7,157.5
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	6.7
Amortized cost, Due after one through five years	137.0
Amortized cost, Due after five through ten years	4.7
Amortized cost, Due after ten years	15.1
Debt Securities, Amortized cost	163.5	203.3
Estimated fair value, Due in one year or less	6.7
Estimated fair value, Due after one through five years	132.3
Estimated fair value, Due after five through ten years	4.7
Estimated fair value, Due after ten years	13.8
Debt securities, Estimated fair value	157.5	199.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	13.1
Amortized cost, Due after one through five years	403.4
Amortized cost, Due after five through ten years	462.0
Amortized cost, Due after ten years	503.8
Debt Securities, Amortized cost	1,382.3	1,373.7
Estimated fair value, Due in one year or less	12.9
Estimated fair value, Due after one through five years	370.2
Estimated fair value, Due after five through ten years	393.3
Estimated fair value, Due after ten years	468.6
Debt securities, Estimated fair value	1,245.0	1,245.8
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	8.9
Amortized cost, Due after one through five years	139.2
Amortized cost, Due after five through ten years	68.7
Amortized cost, Due after ten years	6.6
Debt Securities, Amortized cost	223.4	228.4
Estimated fair value, Due in one year or less	8.8
Estimated fair value, Due after one through five years	138.9
Estimated fair value, Due after five through ten years	60.2
Estimated fair value, Due after ten years	5.8
Debt securities, Estimated fair value	213.7	219.3
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	36.3
Amortized cost, Due after one through five years	130.3
Amortized cost, Due after five through ten years	1.5
Amortized cost, Due after ten years	45.6
Debt Securities, Amortized cost	213.7	207.7
Estimated fair value, Due in one year or less	35.6
Estimated fair value, Due after one through five years	126.1
Estimated fair value, Due after five through ten years	1.4
Estimated fair value, Due after ten years	36.7
Debt securities, Estimated fair value	199.8	195.4
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	18.7
Amortized cost, Due after one through five years	723.7
Amortized cost, Due after five through ten years	272.6
Amortized cost, Due after ten years	80.7
Debt Securities, Amortized cost	1,095.7	1,007.0
Estimated fair value, Due in one year or less	18.6
Estimated fair value, Due after one through five years	686.5
Estimated fair value, Due after five through ten years	266.5
Estimated fair value, Due after ten years	70.0
Debt securities, Estimated fair value	1,041.6	958.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	28.7
Amortized cost, Due after one through five years	312.5
Amortized cost, Due after five through ten years	125.0
Amortized cost, Due after ten years	32.0
Debt Securities, Amortized cost	498.2	478.9
Estimated fair value, Due in one year or less	28.2
Estimated fair value, Due after one through five years	302.5
Estimated fair value, Due after five through ten years	121.9
Estimated fair value, Due after ten years	28.4
Debt securities, Estimated fair value	481.0	$ 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	112.4
Amortized cost, Due after one through five years	1,846.1
Amortized cost, Due after five through ten years	934.5
Amortized cost, Due after ten years	683.8
Debt Securities, Amortized cost	3,576.8
Estimated fair value, Due in one year or less	110.8
Estimated fair value, Due after one through five years	1,756.5
Estimated fair value, Due after five through ten years	848.0
Estimated fair value, Due after ten years	623.3
Debt securities, Estimated fair value	3,338.6
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	4,315.9
Estimated fair value	$ 3,742.1