Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 11,259.5	$ 12,016.8	$ 11,688.3
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)	431.3	(1,533.6)	(463.6)
Balance, end of period	11,897.9	11,259.5	12,016.8
TOTAL
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(1,569.2)	(35.6)	428.0
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)	431.3	(1,533.6)	(463.6)
Balance, end of period	(1,137.9)	(1,569.2)	(35.6)
NET UNREALIZED GAINS (LOSSES) ON AVAILABLE FOR SALE DEBT SECURITIES
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(1,367.6)	107.1	641.8
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)	443.7	(1,474.7)	(534.7)
Balance, end of period	(923.9)	(1,367.6)	107.1
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	1.2	(2.4)	(3.2)
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)	(0.4)	3.6	0.8
Balance, end of period	0.8	1.2	(2.4)
NET FOREIGN CURRENCY ADJUSTMENT
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	164.6	155.2	144.7
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)	39.0	9.4	10.5
Balance, end of period	203.6	164.6	155.2
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(367.4)	(295.5)	(355.3)
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)	(51.0)	(71.9)	59.8
Balance, end of period	$ (418.4)	$ (367.4)	$ (295.5)