Stockholders' Equity - Schedule of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 7,947.2	$ 6,533.9
Other comprehensive income before reclassifications	71.6	29.5
Amounts reclassified from AOCL	2.6	2.7
Current period other comprehensive income	74.2	32.2
Ending Balance	7,725.5	6,621.2
Pension and Other Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(176.2)	(192.5)
Other comprehensive income before reclassifications	0.0	0.0
Amounts reclassified from AOCL	1.6	1.8
Current period other comprehensive income	1.6	1.8
Ending Balance	(174.6)	(190.7)
Net Unrealized Gains (Losses) on Debt Securities Available-for-Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	20.8	(47.0)
Other comprehensive income before reclassifications	71.6	29.0
Amounts reclassified from AOCL	0.0	0.0
Current period other comprehensive income	71.6	29.0
Ending Balance	92.4	(18.0)
Net Unrealized Gains (Losses) on Debt Securities Transferred to Held-to-Maturity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(11.8)	(15.3)
Other comprehensive income before reclassifications	0.0	0.0
Amounts reclassified from AOCL	1.0	0.6
Current period other comprehensive income	1.0	0.6
Ending Balance	(10.8)	(14.7)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.3	(2.0)
Other comprehensive income before reclassifications	0.0	0.5
Amounts reclassified from AOCL	0.0	0.3
Current period other comprehensive income	0.0	0.8
Ending Balance	0.3	(1.2)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(166.9)	(256.8)
Current period other comprehensive income	74.2	32.2
Ending Balance	$ (92.7)	$ (224.6)