Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 927.5	$ 552.0
Unrealized losses, Less than 12 months	(10.2)	(8.2)
Estimated fair value, 12 months or longer	5,387.4	5,327.6
Unrealized losses, 12 months or longer	(817.4)	(759.1)
Estimated fair value, Total	6,314.9	5,879.6
Unrealized losses, Total	(827.6)	(767.3)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	94.9	8.2
Unrealized losses, Less than 12 months	(1.6)	(0.1)
Estimated fair value, 12 months or longer	55.7	55.4
Unrealized losses, 12 months or longer	(4.5)	(4.4)
Estimated fair value, Total	150.6	63.6
Unrealized losses, Total	(6.1)	(4.5)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	99.3	107.4
Unrealized losses, Less than 12 months	(1.4)	(0.9)
Estimated fair value, 12 months or longer	1,010.8	956.8
Unrealized losses, 12 months or longer	(141.8)	(135.8)
Estimated fair value, Total	1,110.1	1,064.2
Unrealized losses, Total	(143.2)	(136.7)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	57.3	33.3
Unrealized losses, Less than 12 months	(0.1)	(0.1)
Estimated fair value, 12 months or longer	81.8	101.4
Unrealized losses, 12 months or longer	(10.0)	(10.4)
Estimated fair value, Total	139.1	134.7
Unrealized losses, Total	(10.1)	(10.5)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	82.5	0.4
Unrealized losses, Less than 12 months	(0.7)	0.0
Estimated fair value, 12 months or longer	117.3	118.9
Unrealized losses, 12 months or longer	(13.2)	(12.5)
Estimated fair value, Total	199.8	119.3
Unrealized losses, Total	(13.9)	(12.5)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	363.7	338.3
Unrealized losses, Less than 12 months	(3.7)	(6.6)
Estimated fair value, 12 months or longer	3,283.7	3,225.3
Unrealized losses, 12 months or longer	(571.7)	(520.2)
Estimated fair value, Total	3,647.4	3,563.6
Unrealized losses, Total	(575.4)	(526.8)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	161.3	45.1
Unrealized losses, Less than 12 months	(1.9)	(0.4)
Estimated fair value, 12 months or longer	587.9	602.5
Unrealized losses, 12 months or longer	(56.2)	(54.8)
Estimated fair value, Total	749.2	647.6
Unrealized losses, Total	(58.1)	(55.2)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	68.5	19.3
Unrealized losses, Less than 12 months	(0.8)	(0.1)
Estimated fair value, 12 months or longer	250.2	267.3
Unrealized losses, 12 months or longer	(20.0)	(21.0)
Estimated fair value, Total	318.7	286.6
Unrealized losses, Total	$ (20.8)	$ (21.1)