Allowance for Credit Losses and Credit Quality Information - Schedule of Allowance for Loan Losses and Loan Balances (Details) - 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	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for credit losses
Beginning Balance				$ 47,576				$ 39,539	$ 47,576	$ 39,539
Charge-offs									(13,521)
Recoveries									5,714
Provision for credit losses	$ (936)	$ 2,716	$ 94,754	56,646	$ 1,590	$ 4,121	$ 12,195	7,654	153,180	25,560	$ 13,170
Ending balance	228,804				47,576				228,804	47,576	39,539
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				47,576				39,539	47,576	39,539	40,599
Charge-offs										(22,273)	(16,983)
Recoveries										4,750	2,753
Provision (credit)										24,473	13,277
Provision (credit) for acquired loans										1,087	(107)
Ending balance					47,576					47,576	39,539
Loans evaluated on an individual basis												$ 14	$ 1,841	$ 2,031
Loans evaluated on a collective basis												228,790	45,335	37,282
Acquired loans evaluated for impairment										400	226
Ending balance	228,804			47,576	47,576			39,539	228,804	39,539	39,539	228,804	47,576	39,539
Loans individually evaluated for impairment												49,224	36,589	45,007
Loans evaluated on a collective basis												8,965,453	5,179,722	4,404,324
Acquired nonimpaired loans													3,221,316	429,982
Acquired impaired loans										26,935	14,944
Ending balance												9,014,677	8,464,562
Reverse mortgage, fair value												10,100	16,600	16,500
Initial allowance on loans purchased with credit deterioration									100
Performing TDRs												15,539	14,281	15,000
Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				35,855					35,855
Ending balance					35,855					35,855
Allowance for loan and lease losses
Ending balance				35,855	35,855				35,855	35,855			35,855
Total Commercial
Allowance for loan and lease losses
Acquired nonimpaired loans													2,200,000
Total Commercial | Commercial and industrial(1)
Allowance for credit losses
Beginning Balance				22,849				14,211	22,849	14,211
Charge-offs									(10,388)
Recoveries									4,255
Provision for credit losses									114,412
Ending balance	150,875				22,849				150,875	22,849	14,211
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				22,849				14,211	22,849	14,211	16,732
Charge-offs										(17,258)	(12,130)
Recoveries										1,621	1,381
Provision (credit)										23,977	8,328
Provision (credit) for acquired loans										298	(100)
Ending balance					22,849					22,849	14,211
Loans evaluated on an individual basis												1	1,179	876
Loans evaluated on a collective basis												150,874	21,664	13,334
Acquired loans evaluated for impairment										6	1
Ending balance	150,875			22,849	22,849			14,211	22,849	14,211	14,211	150,875	22,849	14,211
Loans individually evaluated for impairment												14,048	11,158	14,837
Loans evaluated on a collective basis												2,935,255	1,619,549	1,366,151
Acquired nonimpaired loans													603,157	89,970
Acquired impaired loans										1,564	1,531
Ending balance												2,949,303	2,235,428
Total Commercial | Commercial and industrial(1) | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				19,747					19,747
Ending balance					19,747					19,747
Allowance for loan and lease losses
Ending balance				19,747	19,747				19,747	19,747			19,747
Total Commercial | Owner-occupied commercial
Allowance for credit losses
Beginning Balance				4,616				5,057	4,616	5,057
Charge-offs									(336)
Recoveries									142
Provision for credit losses									6,665
Ending balance	9,615				4,616				9,615	4,616	5,057
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				4,616				5,057	4,616	5,057	5,422
Charge-offs										(354)	(417)
Recoveries										200	34
Provision (credit)										(472)	(38)
Provision (credit) for acquired loans										185	56
Ending balance					4,616					4,616	5,057
Loans evaluated on an individual basis												0	23	0
Loans evaluated on a collective basis												9,615	4,383	4,965
Acquired loans evaluated for impairment										210	92
Ending balance	9,615			4,616	4,616			5,057	9,615	5,057	5,057	9,615	4,616	5,057
Loans individually evaluated for impairment												6,496	4,060	4,406
Loans evaluated on a collective basis												1,326,231	971,694	938,934
Acquired nonimpaired loans													313,955	112,386
Acquired impaired loans										6,757	4,248
Ending balance												1,332,727	1,296,466
Total Commercial | Owner-occupied commercial | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				(1,472)					(1,472)
Ending balance					(1,472)					(1,472)
Allowance for loan and lease losses
Ending balance				(1,472)	(1,472)				(1,472)	(1,472)			(1,472)
Total Commercial | Commercial mortgages
Allowance for credit losses
Beginning Balance				7,452				6,806	7,452	6,806
Charge-offs									(104)
Recoveries									158
Provision for credit losses									21,903
Ending balance	31,071				7,452				31,071	7,452	6,806
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				7,452				6,806	7,452	6,806	5,891
Charge-offs										(159)	(255)
Recoveries										1,546	255
Provision (credit)										(830)	924
Provision (credit) for acquired loans										89	(9)
Ending balance					7,452					7,452	6,806
Loans evaluated on an individual basis												13	0	0
Loans evaluated on a collective basis												31,058	7,387	6,727
Acquired loans evaluated for impairment										65	79
Ending balance	31,071			7,452	7,452			6,806	31,071	6,806	6,806	31,071	7,452	6,806
Loans individually evaluated for impairment												20,309	1,753	4,083
Loans evaluated on a collective basis												2,065,753	1,105,174	1,005,504
Acquired nonimpaired loans													1,107,379	145,648
Acquired impaired loans										8,670	7,504
Ending balance												2,086,062	2,222,976
Total Commercial | Commercial mortgages | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				1,662					1,662
Ending balance					1,662					1,662
Allowance for loan and lease losses
Ending balance				1,662	1,662				1,662	1,662			1,662
Total Commercial | Construction
Allowance for credit losses
Beginning Balance				3,891				3,712	3,891	3,712
Charge-offs									0
Recoveries									36
Provision for credit losses									7,582
Ending balance	12,190				3,891				12,190	3,891	3,712
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				3,891				3,712	3,891	3,712	2,861
Charge-offs										(42)	(1,475)
Recoveries										4	3
Provision (credit)										207	2,341
Provision (credit) for acquired loans										10	(18)
Ending balance					3,891					3,891	3,712
Loans evaluated on an individual basis												0	0	444
Loans evaluated on a collective basis												12,190	3,867	3,254
Acquired loans evaluated for impairment										24	14
Ending balance	12,190			3,891	3,891			3,712	12,190	3,712	3,712	12,190	3,891	3,712
Loans individually evaluated for impairment												79	0	2,781
Loans evaluated on a collective basis												716,196	437,999	310,511
Acquired nonimpaired loans													142,592	2,525
Acquired impaired loans										491	749
Ending balance												716,275	581,082
Total Commercial | Construction | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				681					681
Ending balance					681					681
Allowance for loan and lease losses
Ending balance				681	681				681	681			681
Residential
Allowance for credit losses
Beginning Balance				1,381				1,428	1,381	1,428
Charge-offs									(229)
Recoveries									230
Provision for credit losses									(2,011)
Ending balance	6,893				1,381				6,893	1,381	1,428
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				1,381				1,428	1,381	1,428	1,798
Charge-offs										(322)	(91)
Recoveries										(84)	154
Provision (credit)										126	(404)
Provision (credit) for acquired loans										233	(29)
Ending balance					1,381					1,381	1,428
Loans evaluated on an individual basis												0	463	543
Loans evaluated on a collective basis												6,893	824	847
Acquired loans evaluated for impairment										94	38
Ending balance	6,893			1,381	1,381			1,428	1,381	1,428	1,428	6,893	1,381	1,428
Loans individually evaluated for impairment												5,921	12,151	11,017
Loans evaluated on a collective basis												758,472	145,582	132,064
Acquired nonimpaired loans													834,820	57,708
Acquired impaired loans										7,326	761
Ending balance												764,393	999,879
Residential | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				7,522					7,522
Ending balance					7,522					7,522
Allowance for loan and lease losses
Ending balance				7,522	7,522				7,522	7,522			7,522
Consumer
Allowance for credit losses
Beginning Balance				7,387				8,325	7,387	8,325
Charge-offs									(2,464)
Recoveries									893
Provision for credit losses									4,629
Ending balance	18,160				7,387				18,160	7,387	8,325
Allowance for loan and lease losses
Beginning balance, prior to adoption of ASC 326				7,387				8,325	7,387	8,325	7,895
Charge-offs										(4,138)	(2,615)
Recoveries										1,463	926
Provision (credit)										1,465	2,126
Provision (credit) for acquired loans										272	(7)
Ending balance					7,387					7,387	8,325
Loans evaluated on an individual basis												0	176	168
Loans evaluated on a collective basis												18,160	7,210	8,155
Acquired loans evaluated for impairment										1	2
Ending balance	$ 18,160			7,387	7,387			$ 8,325	7,387	8,325	8,325	18,160	7,387	8,325
Loans individually evaluated for impairment												2,371	7,467	7,883
Loans evaluated on a collective basis												1,163,546	899,724	651,160
Acquired nonimpaired loans													219,413	$ 21,745
Acquired impaired loans										2,127	$ 151
Ending balance												$ 1,165,917	1,128,731
Consumer | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance				7,715					7,715
Ending balance					7,715					7,715
Allowance for loan and lease losses
Ending balance				$ 7,715	$ 7,715				$ 7,715	$ 7,715			$ 7,715