Allowance for Loan and Lease Losses and Credit Quality Information - Schedule of Allowance for Loan Losses and Loan Balances (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 39,539	$ 40,599	$ 39,751
Charge-offs	(22,273)	(16,983)	(13,842)
Recoveries	4,750	2,753	3,726
Provision (credit)	24,473	13,277	10,528
Provision (credit) for acquired loans	1,087	(107)	436
Ending balance	47,576	39,539	40,599
Individually evaluated for impairment	1,841	2,031	4,658
Collectively evaluated for impairment	45,335	37,282	35,583
Acquired loans evaluated for impairment	400	226	358
Ending balance	47,576	39,539	40,599
Loans individually evaluated for impairment	36,589	45,007	56,315
Collectively evaluated for impairment	5,179,722	4,404,324	4,161,609
Acquired nonimpaired loans	3,221,316	429,982	565,516
Acquired impaired loans	26,935	14,944	21,654
Ending balance	8,464,562	4,894,257	4,805,094
Reverse mortgage, fair value	16,600	16,500	19,800
Performing TDRs	14,281	14,953	20,100
Owner-occupied commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,057	5,422	6,588
Charge-offs	(354)	(417)	(296)
Recoveries	200	34	127
Provision (credit)	(472)	(38)	(1,098)
Provision (credit) for acquired loans	185	56	101
Ending balance	4,616	5,057	5,422
Individually evaluated for impairment	23	0	0
Collectively evaluated for impairment	4,383	4,965	5,410
Acquired loans evaluated for impairment	210	92	12
Ending balance	4,616	5,057	5,422
Loans individually evaluated for impairment	4,060	4,406	3,655
Collectively evaluated for impairment	971,694	938,934	933,352
Acquired nonimpaired loans	313,955	112,386	136,437
Acquired impaired loans	6,757	4,248	5,803
Ending balance	1,296,466	1,059,974	1,079,247
Commercial mortgages
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,806	5,891	8,915
Charge-offs	(159)	(255)	(4,612)
Recoveries	1,546	255	255
Provision (credit)	(830)	924	1,160
Provision (credit) for acquired loans	89	(9)	173
Ending balance	7,452	6,806	5,891
Individually evaluated for impairment	0	0	18
Collectively evaluated for impairment	7,387	6,727	5,779
Acquired loans evaluated for impairment	65	79	94
Ending balance	7,452	6,806	5,891
Loans individually evaluated for impairment	1,753	4,083	6,076
Collectively evaluated for impairment	1,105,174	1,005,504	983,400
Acquired nonimpaired loans	1,107,379	145,648	188,505
Acquired impaired loans	8,670	7,504	9,724
Ending balance	2,222,976	1,162,739	1,187,705
Construction
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	3,712	2,861	2,838
Charge-offs	(42)	(1,475)	(574)
Recoveries	4	3	306
Provision (credit)	207	2,341	222
Provision (credit) for acquired loans	10	(18)	69
Ending balance	3,891	3,712	2,861
Individually evaluated for impairment	0	444	0
Collectively evaluated for impairment	3,867	3,254	2,828
Acquired loans evaluated for impairment	24	14	33
Ending balance	3,891	3,712	2,861
Loans individually evaluated for impairment	0	2,781	6,022
Collectively evaluated for impairment	437,999	310,511	258,887
Acquired nonimpaired loans	142,592	2,525	15,759
Acquired impaired loans	491	749	940
Ending balance	581,082	316,566	281,608
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	14,211	16,732	13,339
Charge-offs	(17,258)	(12,130)	(5,008)
Recoveries	1,621	1,381	1,355
Provision (credit)	23,977	8,328	6,972
Provision (credit) for acquired loans	298	(100)	74
Ending balance	22,849	14,211	16,732
Individually evaluated for impairment	1,179	876	3,687
Collectively evaluated for impairment	21,664	13,334	12,871
Acquired loans evaluated for impairment	6	1	174
Ending balance	22,849	14,211	16,732
Loans individually evaluated for impairment	11,158	14,837	19,196
Collectively evaluated for impairment	1,619,549	1,366,151	1,324,636
Acquired nonimpaired loans	603,157	89,970	116,566
Acquired impaired loans	1,564	1,531	4,156
Ending balance	2,235,428	1,472,489	1,464,554
Residential
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,428	1,798	2,059
Charge-offs	(322)	(91)	(168)
Recoveries	(84)	154	178
Provision (credit)	126	(404)	(300)
Provision (credit) for acquired loans	233	(29)	29
Ending balance	1,381	1,428	1,798
Individually evaluated for impairment	463	543	760
Collectively evaluated for impairment	824	847	1,002
Acquired loans evaluated for impairment	94	38	36
Ending balance	1,381	1,428	1,798
Loans individually evaluated for impairment	12,151	11,017	13,778
Collectively evaluated for impairment	145,582	132,064	146,621
Acquired nonimpaired loans	834,820	57,708	72,304
Acquired impaired loans	7,326	761	784
Ending balance	999,879	201,550	233,487
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,325	7,895	6,012
Charge-offs	(4,138)	(2,615)	(3,184)
Recoveries	1,463	926	1,505
Provision (credit)	1,465	2,126	3,572
Provision (credit) for acquired loans	272	(7)	(10)
Ending balance	7,387	8,325	7,895
Individually evaluated for impairment	176	168	193
Collectively evaluated for impairment	7,210	8,155	7,693
Acquired loans evaluated for impairment	1	2	9
Ending balance	7,387	8,325	7,895
Loans individually evaluated for impairment	7,467	7,883	7,588
Collectively evaluated for impairment	899,724	651,160	514,713
Acquired nonimpaired loans	219,413	21,745	35,945
Acquired impaired loans	2,127	151	247
Ending balance	$ 1,128,731	$ 680,939	$ 558,493