Business Segments - Financial Information by Business Segment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Interest and dividend income									$ 541,277	$ 484,253	$ 414,957
Interest expense									61,797	84,349	67,721
Net interest income									479,480	399,904	347,236
Provision for credit losses	$ (209)	$ 16,288	$ 61,841	$ 29,821	$ 1,824	$ 4,121	$ 2,747	$ 2,507	107,741	11,198	7,764
Net interest income after provision for credit losses									371,739	388,706	339,472
Non-interest income									128,185	116,716	100,276
Non-interest expense	88,069	89,943	85,502	91,333	92,556	73,268	71,952	74,432	354,845	312,208	265,224
Income before provision for income taxes	64,557	48,974	4,530	27,017	43,422	45,681	54,917	49,195	145,079	193,214	174,524
Provision for income taxes	11,703	7,669	42	3,621	7,046	8,334	10,103	8,858	23,035	34,341	31,412
NET INCOME	52,854	41,305	4,488	23,396	$ 36,376	$ 37,347	$ 44,814	$ 40,337	122,044	158,873	143,112
Preferred stock dividends	2,644								2,644
NET INCOME AVAILABLE TO COMMON SHAREHOLDERS	$ 50,210	$ 41,305	$ 4,488	$ 23,396					119,400	158,873	143,112
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest and dividend income									541,277	484,253	414,957
Interest expense									61,797	84,349	67,721
Net interest income									479,480	399,904	347,236
Provision for credit losses									107,741	11,198	7,764
Net interest income after provision for credit losses									371,739	388,706	339,472
Non-interest income									101,850	90,137	75,653
Non-interest expense									338,526	295,747	250,338
Income before provision for income taxes									135,063	183,096	164,787
Provision for income taxes									20,932	32,216	29,367
NET INCOME									114,131
Preferred stock dividends									2,644
NET INCOME AVAILABLE TO COMMON SHAREHOLDERS									111,487	150,880	135,420
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income									26,335	26,579	24,623
Non-interest expense									16,319	16,461	14,886
Income before provision for income taxes									10,016	10,118	9,737
Provision for income taxes									2,103	2,125	2,045
NET INCOME									7,913
NET INCOME AVAILABLE TO COMMON SHAREHOLDERS									$ 7,913	$ 7,993	$ 7,692