Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,413.5	$ 9,169.6
Debt securities, Estimated fair value	7,454.5	8,169.6
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	13.9
Amortized cost, Due after one through five years	109.3
Amortized cost, Due after five through ten years	3.1
Amortized cost, Due after ten years	4.6
Debt Securities, Amortized cost	130.9	308.5
Estimated fair value, Due in one year or less	13.7
Estimated fair value, Due after one through five years	103.6
Estimated fair value, Due after five through ten years	2.7
Estimated fair value, Due after ten years	4.4
Debt securities, Estimated fair value	124.4	302.8
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	11.3
Amortized cost, Due after one through five years	282.2
Amortized cost, Due after five through ten years	547.2
Amortized cost, Due after ten years	558.4
Debt Securities, Amortized cost	1,399.1	1,670.6
Estimated fair value, Due in one year or less	11.2
Estimated fair value, Due after one through five years	257.0
Estimated fair value, Due after five through ten years	471.3
Estimated fair value, Due after ten years	500.5
Debt securities, Estimated fair value	1,240.0	1,478.7
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	40.6
Amortized cost, Due after one through five years	105.5
Amortized cost, Due after five through ten years	74.1
Amortized cost, Due after ten years	9.3
Debt Securities, Amortized cost	229.5	208.0
Estimated fair value, Due in one year or less	40.2
Estimated fair value, Due after one through five years	103.4
Estimated fair value, Due after five through ten years	64.2
Estimated fair value, Due after ten years	8.1
Debt securities, Estimated fair value	215.9	193.7
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	25.0
Amortized cost, Due after one through five years	158.3
Amortized cost, Due after five through ten years	6.5
Amortized cost, Due after ten years	52.5
Debt Securities, Amortized cost	242.3	247.9
Estimated fair value, Due in one year or less	24.7
Estimated fair value, Due after one through five years	151.5
Estimated fair value, Due after five through ten years	5.8
Estimated fair value, Due after ten years	42.5
Debt securities, Estimated fair value	224.5	228.6
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	7.0
Amortized cost, Due after one through five years	709.2
Amortized cost, Due after five through ten years	194.4
Amortized cost, Due after ten years	62.1
Debt Securities, Amortized cost	972.7	1,004.4
Estimated fair value, Due in one year or less	6.9
Estimated fair value, Due after one through five years	655.1
Estimated fair value, Due after five through ten years	183.9
Estimated fair value, Due after ten years	51.8
Debt securities, Estimated fair value	897.7	921.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	16.9
Amortized cost, Due after one through five years	317.3
Amortized cost, Due after five through ten years	100.0
Amortized cost, Due after ten years	36.7
Debt Securities, Amortized cost	470.9	476.8
Estimated fair value, Due in one year or less	16.6
Estimated fair value, Due after one through five years	297.4
Estimated fair value, Due after five through ten years	94.2
Estimated fair value, Due after ten years	32.4
Debt securities, Estimated fair value	440.6	$ 442.0
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	114.7
Amortized cost, Due after one through five years	1,681.8
Amortized cost, Due after five through ten years	925.3
Amortized cost, Due after ten years	723.6
Debt Securities, Amortized cost	3,445.4
Estimated fair value, Due in one year or less	113.3
Estimated fair value, Due after one through five years	1,568.0
Estimated fair value, Due after five through ten years	822.1
Estimated fair value, Due after ten years	639.7
Debt securities, Estimated fair value	3,143.1
Collateralized Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	4,968.1
Estimated fair value	$ 4,311.4