Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,067.8	$ 552.0
Unrealized losses, Less than 12 months	(21.2)	(8.2)
Estimated fair value, 12 months or longer	5,194.2	5,327.6
Unrealized losses, 12 months or longer	(801.3)	(759.1)
Estimated fair value, Total	6,262.0	5,879.6
Unrealized losses, Total	(822.5)	(767.3)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	101.2	8.2
Unrealized losses, Less than 12 months	(1.3)	(0.1)
Estimated fair value, 12 months or longer	50.4	55.4
Unrealized losses, 12 months or longer	(4.6)	(4.4)
Estimated fair value, Total	151.6	63.6
Unrealized losses, Total	(5.9)	(4.5)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	133.1	107.4
Unrealized losses, Less than 12 months	(1.5)	(0.9)
Estimated fair value, 12 months or longer	970.8	956.8
Unrealized losses, 12 months or longer	(137.5)	(135.8)
Estimated fair value, Total	1,103.9	1,064.2
Unrealized losses, Total	(139.0)	(136.7)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	68.3	33.3
Unrealized losses, Less than 12 months	(0.2)	(0.1)
Estimated fair value, 12 months or longer	96.1	101.4
Unrealized losses, 12 months or longer	(10.5)	(10.4)
Estimated fair value, Total	164.4	134.7
Unrealized losses, Total	(10.7)	(10.5)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	83.0	0.4
Unrealized losses, Less than 12 months	(0.6)	0.0
Estimated fair value, 12 months or longer	117.4	118.9
Unrealized losses, 12 months or longer	(13.3)	(12.5)
Estimated fair value, Total	200.4	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	490.3	338.3
Unrealized losses, Less than 12 months	(15.4)	(6.6)
Estimated fair value, 12 months or longer	3,103.7	3,225.3
Unrealized losses, 12 months or longer	(557.6)	(520.2)
Estimated fair value, Total	3,594.0	3,563.6
Unrealized losses, Total	(573.0)	(526.8)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	127.1	45.1
Unrealized losses, Less than 12 months	(1.6)	(0.4)
Estimated fair value, 12 months or longer	598.1	602.5
Unrealized losses, 12 months or longer	(57.0)	(54.8)
Estimated fair value, Total	725.2	647.6
Unrealized losses, Total	(58.6)	(55.2)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	64.8	19.3
Unrealized losses, Less than 12 months	(0.6)	(0.1)
Estimated fair value, 12 months or longer	257.7	267.3
Unrealized losses, 12 months or longer	(20.8)	(21.0)
Estimated fair value, Total	322.5	286.6
Unrealized losses, Total	$ (21.4)	$ (21.1)