Acquisitions and Dispositions - Summary of Pro Forma Financial Information Reflecting Acquisition (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Selected Financial Results:
Net interest income	$ 1,582.5	$ 1,488.6
Provision for loan losses	28.3	30.0
Non-interest income	454.5	408.1
Non-interest expense	1,277.5	1,186.4
Net income	583.5	552.4
Net income available to common shareholders	$ 569.4	$ 538.3
EPS:
Basic (in dollars per share)	$ 1.31	$ 1.30
Diluted (in dollars per share)	$ 1.30	$ 1.29