Comprehensive Income - Schedule of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 6,533.9	$ 5,819.9	$ 5,141.9
Other comprehensive income (loss) before reclassifications	79.1	(55.0)	(8.7)
Amounts reclassified from AOCL	10.8	17.2	22.0
Total other comprehensive income (loss), net of tax	89.9	(37.8)	13.3
Transition adjustments related to adoption of new accounting standards				$ 0.6
Ending Balance	7,947.2	6,533.9	5,819.9
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(256.8)	(181.7)	(195.0)
Total other comprehensive income (loss), net of tax	89.9	(37.8)	13.3
Transition adjustments related to adoption of new accounting standards				(37.3)
Ending Balance	(166.9)	(256.8)	(181.7)
Pension and Other Postretirement Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(192.5)	(144.1)	(145.6)
Other comprehensive income (loss) before reclassifications	9.7	(24.7)	(2.8)
Amounts reclassified from AOCL	6.6	6.3	4.3
Total other comprehensive income (loss), net of tax	16.3	(18.4)	1.5
Transition adjustments related to adoption of new accounting standards				(30.0)
Ending Balance	(176.2)	(192.5)	(144.1)
Net Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(47.0)	(21.6)	(32.3)
Other comprehensive income (loss) before reclassifications	67.9	(29.0)	(5.3)
Amounts reclassified from AOCL	(0.1)	7.5	16.0
Total other comprehensive income (loss), net of tax	67.8	(21.5)	10.7
Transition adjustments related to adoption of new accounting standards				(3.9)
Ending Balance	20.8	(47.0)	(21.6)
Net Unrealized Gain (Losses) On Debt Securities Transferred To Held To Maturity [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(15.3)	(15.1)	(17.4)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0
Amounts reclassified from AOCL	3.5	3.0	2.3
Total other comprehensive income (loss), net of tax	3.5	3.0	2.3
Transition adjustments related to adoption of new accounting standards				(3.2)
Ending Balance	(11.8)	(15.3)	(15.1)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2.0)	(0.9)	0.3
Other comprehensive income (loss) before reclassifications		(1.3)	(0.6)
Amounts reclassified from AOCL		0.4	(0.6)
Total other comprehensive income (loss), net of tax		(0.9)	(1.2)
Transition adjustments related to adoption of new accounting standards				$ (0.2)
Ending Balance		$ (2.0)	$ (0.9)
Net Unrealized Gains (Losses) on Derivatives Accounted for as Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications	1.5
Amounts reclassified from AOCL	0.8
Total other comprehensive income (loss), net of tax	2.3
Ending Balance	$ 0.3