Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,859.0	$ 7,895.2
Debt securities, Estimated fair value	7,054.4	7,157.5
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	5.7
Amortized cost, Due after one through five years	136.9
Amortized cost, Due after five through ten years	0.4
Amortized cost, Due after ten years	20.1
Debt Securities, Amortized cost	163.1	203.3
Estimated fair value, Due in one year or less	5.7
Estimated fair value, Due after one through five years	132.1
Estimated fair value, Due after five through ten years	0.3
Estimated fair value, Due after ten years	19.1
Debt securities, Estimated fair value	157.2	199.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	12.6
Amortized cost, Due after one through five years	382.4
Amortized cost, Due after five through ten years	478.4
Amortized cost, Due after ten years	507.8
Debt Securities, Amortized cost	1,381.2	1,373.7
Estimated fair value, Due in one year or less	12.5
Estimated fair value, Due after one through five years	351.1
Estimated fair value, Due after five through ten years	413.1
Estimated fair value, Due after ten years	472.6
Debt securities, Estimated fair value	1,249.3	1,245.8
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	31.2
Amortized cost, Due after one through five years	104.5
Amortized cost, Due after five through ten years	67.5
Amortized cost, Due after ten years	6.7
Debt Securities, Amortized cost	209.9	228.4
Estimated fair value, Due in one year or less	31.0
Estimated fair value, Due after one through five years	103.8
Estimated fair value, Due after five through ten years	58.6
Estimated fair value, Due after ten years	6.0
Debt securities, Estimated fair value	199.4	219.3
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	0.0
Amortized cost, Due after one through five years	166.6
Amortized cost, Due after five through ten years	1.6
Amortized cost, Due after ten years	46.1
Debt Securities, Amortized cost	214.3	207.7
Estimated fair value, Due in one year or less	0.0
Estimated fair value, Due after one through five years	161.6
Estimated fair value, Due after five through ten years	1.5
Estimated fair value, Due after ten years	37.3
Debt securities, Estimated fair value	200.4	195.4
U.S. corporate debt securities
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	732.1
Amortized cost, Due after five through ten years	245.6
Amortized cost, Due after ten years	83.8
Debt Securities, Amortized cost	1,075.4	1,007.0
Estimated fair value, Due in one year or less	13.8
Estimated fair value, Due after one through five years	693.1
Estimated fair value, Due after five through ten years	239.8
Estimated fair value, Due after ten years	74.5
Debt securities, Estimated fair value	1,021.2	958.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	31.9
Amortized cost, Due after one through five years	313.5
Amortized cost, Due after five through ten years	121.2
Amortized cost, Due after ten years	31.8
Debt Securities, Amortized cost	498.4	478.9
Estimated fair value, Due in one year or less	31.3
Estimated fair value, Due after one through five years	302.6
Estimated fair value, Due after five through ten years	118.5
Estimated fair value, Due after ten years	28.6
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	95.3
Amortized cost, Due after one through five years	1,836.0
Amortized cost, Due after five through ten years	914.7
Amortized cost, Due after ten years	696.3
Debt Securities, Amortized cost	3,542.3
Estimated fair value, Due in one year or less	94.3
Estimated fair value, Due after one through five years	1,744.3
Estimated fair value, Due after five through ten years	831.8
Estimated fair value, Due after ten years	638.1
Debt securities, Estimated fair value	3,308.5
Total Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	4,316.7
Estimated fair value	$ 3,745.9