Segment Information - Selected Financial Information Business Segments (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net interest income (loss)	$ 382.7	$ 348.7	$ 348.1	$ 332.8	$ 332.6	$ 306.4	$ 301.2	$ 295.8	$ 1,412.3	$ 1,236.0	$ 1,100.5
Provision for loan losses	7.3	7.8	7.6	5.6	9.9	8.2	6.5	5.4	28.3	30.0	26.0
Total non-interest income	124.2	106.0	106.3	94.6	88.7	92.3	94.9	90.4	431.1	366.4	352.9
Total non-interest expense	325.7	281.4	278.4	277.2	262.7	241.3	248.6	243.5	1,162.7	996.1	960.3
Income before income tax expense	173.9	165.5	168.4	144.6	148.7	149.2	141.0	137.3	652.4	576.3	467.1
Income tax expense (benefit)	36.4	30.4	35.2	30.0	15.8	32.2	30.8	29.4	132.0	108.2	129.9
Net income	$ 137.5	$ 135.1	$ 133.2	$ 114.6	$ 132.9	$ 117.0	$ 110.2	$ 107.9	520.4	468.1	337.2
Average total assets									51,658.0	45,029.7	42,581.6
Average total liabilities									44,586.7	38,992.5	36,990.0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									1,362.2	1,166.4	1,034.9
Provision for loan losses									53.0	47.7	57.1
Total non-interest income									402.4	364.5	348.4
Total non-interest expense									1,048.9	949.0	904.1
Income before income tax expense									662.7	534.2	422.1
Income tax expense (benefit)									132.8	99.9	117.5
Net income									529.9	434.3	304.6
Average total assets									42,307.7	36,060.0	34,229.0
Average total liabilities									34,887.9	30,004.1	28,141.4
Operating Segments [Member] | Commercial Banking Loan [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									807.1	699.2	631.0
Provision for loan losses									44.1	38.7	43.7
Total non-interest income									206.5	177.8	165.0
Total non-interest expense									448.7	383.7	357.1
Income before income tax expense									520.8	454.6	395.2
Income tax expense (benefit)									104.3	85.0	110.0
Net income									416.5	369.6	285.2
Average total assets									29,746.8	25,956.7	24,533.9
Average total liabilities									11,490.2	9,305.0	7,938.6
Operating Segments [Member] | Retail Banking [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									555.1	467.2	403.9
Provision for loan losses									8.9	9.0	13.4
Total non-interest income									195.9	186.7	183.4
Total non-interest expense									600.2	565.3	547.0
Income before income tax expense									141.9	79.6	26.9
Income tax expense (benefit)									28.5	14.9	7.5
Net income									113.4	64.7	19.4
Average total assets									12,560.9	10,103.3	9,695.1
Average total liabilities									23,397.7	20,699.1	20,202.8
Treasury [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									66.5	93.0	107.4
Provision for loan losses									0.0	0.0	0.0
Total non-interest income									14.1	8.3	11.2
Total non-interest expense									13.8	17.8	15.6
Income before income tax expense									66.8	83.5	103.0
Income tax expense (benefit)									13.5	15.8	28.6
Net income									53.3	67.7	74.4
Average total assets									7,882.3	7,955.8	7,512.1
Average total liabilities									9,011.9	8,544.3	8,450.6
Other [Member]
Segment Reporting Information [Line Items]
Net interest income (loss)									(16.4)	(23.4)	(41.8)
Provision for loan losses									(24.7)	(17.7)	(31.1)
Total non-interest income									14.6	(6.4)	(6.7)
Total non-interest expense									100.0	29.3	40.6
Income before income tax expense									(77.1)	(41.4)	(58.0)
Income tax expense (benefit)									(14.3)	(7.5)	(16.2)
Net income									(62.8)	(33.9)	(41.8)
Average total assets									1,468.0	1,013.9	840.5
Average total liabilities									$ 686.9	$ 444.1	$ 398.0