Selected Quarterly Financial Data - Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Quarterly Financial Information [Line Items]
Interest and dividend income	$ 496.9	$ 470.9	$ 472.8	$ 440.5	$ 431.8	$ 390.0	$ 375.4	$ 359.1	$ 1,881.1	$ 1,556.3	$ 1,302.4
Interest expense	114.2	122.2	124.7	107.7	99.2	83.6	74.2	63.3	468.8	320.3	201.9
Net interest income	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
Net interest income after provision for loan losses	375.4	340.9	340.5	327.2	322.7	298.2	294.7	290.4	1,384.0	1,206.0	1,074.5
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
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
Preferred stock dividend	3.5	3.5	3.5	3.5	3.5	3.5	3.5	3.5	14.1	14.1	14.1
Net income available to common shareholders	$ 134.0	$ 131.6	$ 129.7	$ 111.1	$ 129.4	$ 113.5	$ 106.7	$ 104.4	$ 506.3	$ 454.0	$ 323.1
EPS:
Basic (in dollars per share)	$ 0.31	$ 0.34	$ 0.33	$ 0.30	$ 0.35	$ 0.33	$ 0.31	$ 0.31	$ 1.28	$ 1.30	$ 0.98
Diluted (in dollars per share)	$ 0.31	$ 0.33	$ 0.33	$ 0.30	$ 0.35	$ 0.33	$ 0.31	$ 0.30	$ 1.27	$ 1.29	$ 0.97
Common dividends paid	$ 69.9	$ 69.9	$ 69.8	$ 65.2	$ 65.1	$ 60.0	$ 59.9	$ 58.8	$ 274.8	$ 243.8	$ 227.9
Dividends paid per common share (in dollars per share)	$ 0.1775	$ 0.1775	$ 0.1775	$ 0.1750	$ 0.1750	$ 0.1750	$ 0.1750	$ 0.1725	$ 0.1775	$ 0.1750
Common dividend payout ratio	52.20%	53.10%	53.80%	58.60%	50.30%	52.90%	56.20%	56.30%	54.30%	53.70%	70.60%
Basic (in shares)	421,850	391,660	391,270	370,720	370,220	341,430	340,640	339,760	394,000	348,100	330,300
Diluted (in shares)	424,980	394,450	394,570	374,090	372,830	345,040	344,470	344,000	397,200	351,700	332,900
Maximum [Member]
EPS:
Stock price (in dollars per share)	$ 17.22	$ 17.10	$ 17.66	$ 18.03	$ 17.46	$ 19.00	$ 19.37	$ 20.26	$ 17.22	$ 17.46
Minimum [Member]
EPS:
Stock price (in dollars per share)	$ 14.73	$ 13.81	$ 15.24	$ 14.25	$ 13.66	$ 16.95	$ 18.00	$ 18.18	$ 14.73	$ 13.66