Business Segments - Financial Information by Business Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Interest and dividend income	$ 125,054	$ 142,448
Interest expense	8,576	22,286
Net interest income	116,478	120,162
Provision for credit losses	(27,958)	29,821
Net interest income after provision for credit losses	144,436	90,341
Non-interest income	33,208	28,009
Non-interest expense	86,327	91,333
Income before provision for income taxes	91,317	27,017
Provision for income taxes	18,202	3,621
Net income	73,115	23,396
Preferred stock dividends	2,531
Net income available to common shareholders	70,584	23,396
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest and dividend income	125,054	142,448
Interest expense	8,576	22,286
Net interest income	116,478	120,162
Provision for credit losses	(27,958)	29,821
Net interest income after provision for credit losses	144,436	90,341
Non-interest income	25,577	21,057
Non-interest expense	82,011	86,918
Income before provision for income taxes	88,002	24,480
Provision for income taxes	17,506	3,088
Net income	70,496
Preferred stock dividends	2,531
Net income available to common shareholders	67,965	21,392
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income	7,631	6,952
Non-interest expense	4,316	4,415
Income before provision for income taxes	3,315	2,537
Provision for income taxes	696	533
Net income	2,619
Net income available to common shareholders	$ 2,619	$ 2,004