Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	$ 11,091.0	$ 10,508.3	$ 10,216.2
Reclassification of Certain Tax Effects from AOCI			0.0
Net Change	622.7	259.0	(14.1)
Balance, end of period	11,688.3	11,091.0	10,508.3
TOTAL
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(194.7)	(453.7)	(414.3)
Reclassification of Certain Tax Effects from AOCI			(25.3)
Balance, end of period	428.0	(194.7)	(453.7)
NET UNREALIZED GAINS (LOSSES) ON DEBT SECURITIES AVAILABLE FOR SALE
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	114.0	(114.9)	(74.8)
Reclassification of Certain Tax Effects from AOCI			(17.8)
Net Change	527.8	228.9	(22.3)
Balance, end of period	641.8	114.0	(114.9)
NET UNREALIZED (LOSSES) GAINS ON CASH FLOW HEDGES
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(3.7)	4.0	4.5
Reclassification of Certain Tax Effects from AOCI			0.9
Net Change	0.5	(7.7)	(1.4)
Balance, end of period	(3.2)	(3.7)	4.0
NET FOREIGN CURRENCY ADJUSTMENT
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	117.8	67.9	(1.8)
Reclassification of Certain Tax Effects from AOCI			47.5
Net Change	26.9	49.9	22.2
Balance, end of period	144.7	117.8	67.9
NET PENSION AND OTHER POSTRETIREMENT BENEFIT ADJUSTMENTS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	(422.8)	(410.7)	(342.2)
Reclassification of Certain Tax Effects from AOCI			(55.9)
Net Change	67.5	(12.1)	(12.6)
Balance, end of period	$ (355.3)	$ (422.8)	$ (410.7)