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Loans - Summary of Allowance for Loan Losses by Loan Portfolio Segment and Impairment Methodology (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 43,596.1	$ 35,241.4
Allowance Total	246.6	240.4	$ 234.4	$ 229.3
Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment, Portfolio	32,531.1	30,435.1
Individually evaluated for impairment, Portfolio	204.2	214.9
Total loans	32,735.4	30,650.0
Collectively evaluated for impairment, Allowance	234.3	227.3
Individually evaluated for impairment, Allowance	11.2	9.0
PCI [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	396.0	399.9
Allowance Total	0.0	2.3
Purchased Performing [Member] \| Purchased Performing Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment, Portfolio	10,449.7	4,183.5
Individually evaluated for impairment, Portfolio	15.1	8.0
Collectively evaluated for impairment, Allowance	1.1	1.7
Individually evaluated for impairment, Allowance	0.0	0.1
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	30,714.3	25,077.7
Allowance Total	217.9	209.5	204.5	204.9
Commercial [Member] \| Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment, Portfolio	22,938.3	21,900.1
Individually evaluated for impairment, Portfolio	115.0	128.4
Total loans	24,481.7	22,028.5
Collectively evaluated for impairment, Allowance	207.8	198.9
Individually evaluated for impairment, Allowance	9.0	6.7
Allowance Total	216.8	205.6	201.1	198.8
Commercial [Member] \| PCI [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	308.5	300.3
Allowance Total	0.0	2.2
Commercial [Member] \| Purchased Performing [Member] \| Purchased Performing Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment, Portfolio	7,344.9	2,744.4
Individually evaluated for impairment, Portfolio	7.6	4.5
Collectively evaluated for impairment, Allowance	1.1	1.7
Individually evaluated for impairment, Allowance	0.0	0.0
Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	12,881.8	10,163.7
Allowance Total	28.7	30.9	29.9	24.4
Retail Loans [Member] \| Originated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment, Portfolio	9,592.8	8,535.0
Individually evaluated for impairment, Portfolio	89.2	86.5
Total loans	8,253.7	8,621.5
Collectively evaluated for impairment, Allowance	26.5	28.4
Individually evaluated for impairment, Allowance	2.2	2.3
Allowance Total	28.7	30.7	$ 29.7	$ 24.2
Retail Loans [Member] \| PCI [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	87.5	99.6
Allowance Total	0.0	0.1
Retail Loans [Member] \| Purchased Performing [Member] \| Purchased Performing Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment, Portfolio	3,104.8	1,439.1
Individually evaluated for impairment, Portfolio	7.5	3.5
Collectively evaluated for impairment, Allowance	0.0	0.0
Individually evaluated for impairment, Allowance	$ 0.0	$ 0.1