Allowance for Credit Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 44,846	$ 48,833
Accruing Current Balances	8,927,923	8,365,872
Nonaccrual Loans	41,908
Total Loans	9,014,677	8,464,562
Acquired Impaired Loans		26,935
Nonaccrual Loans	$ 13	22,922
Ending balance		$ 8,464,562	$ 4,894,257
Percent past due	0.50%	0.58%
Accruing Current Balances, % of Total Loans	99.04%	98.83%
Percent of Nonaccrual Loans With No Allowance for Credit Losses	0.46%
Acquired Impaired Loans, % of Total Loans		0.32%
Nonaccrual Loans, % of Total Loans		0.27%
% of Total Loans	100.00%
% of Total Loans		100.00%
Reverse mortgage, fair value	$ 10,100	$ 16,600	16,500
Acquired nonimpaired loans		3,221,316	429,982
30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 28,152	$ 32,683
Percent past due	0.31%	0.39%
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 16,694	$ 16,150
Percent past due	0.19%	0.19%
Total Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Acquired nonimpaired loans		$ 2,200,000
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 3,074	7,170
Accruing Current Balances	758,072	980,893
Nonaccrual Loans	3,247
Total Loans	764,393	999,879
Acquired Impaired Loans		7,326
Nonaccrual Loans		4,490
Ending balance		999,879	201,550
Acquired nonimpaired loans		834,820	57,708
Residential | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	3,049	6,733
Residential | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	25	437
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	19,390	25,909
Accruing Current Balances	1,144,217	1,098,980
Nonaccrual Loans	2,310
Total Loans	1,165,917	1,128,731
Acquired Impaired Loans		2,127
Nonaccrual Loans		1,715
Ending balance		1,128,731	680,939
Acquired nonimpaired loans		219,413	21,745
Consumer | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	8,355	13,164
Consumer | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	11,035	12,745
Commercial and industrial(1) | Total Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	10,965	8,327
Accruing Current Balances	2,924,522	2,214,506
Nonaccrual Loans	13,816
Total Loans	2,949,303	2,235,428
Acquired Impaired Loans		1,564
Nonaccrual Loans		11,031
Ending balance		2,235,428	1,472,489
Acquired nonimpaired loans		603,157	89,970
Commercial and industrial(1) | Total Commercial | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	7,313	6,289
Commercial and industrial(1) | Total Commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	3,652	2,038
Owner-occupied commercial | Total Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	4,012	2,329
Accruing Current Balances	1,323,355	1,283,320
Nonaccrual Loans	5,360
Total Loans	1,332,727	1,296,466
Acquired Impaired Loans		6,757
Nonaccrual Loans		4,060
Ending balance		1,296,466	1,059,974
Acquired nonimpaired loans		313,955	112,386
Owner-occupied commercial | Total Commercial | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	3,120	1,498
Owner-occupied commercial | Total Commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	892	831
Commercial mortgages | Total Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	7,034	5,098
Accruing Current Balances	2,061,853	2,207,582
Nonaccrual Loans	17,175
Total Loans	2,086,062	2,222,976
Acquired Impaired Loans		8,670
Nonaccrual Loans		1,626
Ending balance		2,222,976	1,162,739
Acquired nonimpaired loans		1,107,379	145,648
Commercial mortgages | Total Commercial | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	5,944	4,999
Commercial mortgages | Total Commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,090	99
Construction | Total Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	371	0
Accruing Current Balances	715,904	580,591
Nonaccrual Loans	0
Total Loans	716,275	581,082
Acquired Impaired Loans		491
Nonaccrual Loans		0
Ending balance		581,082	316,566
Acquired nonimpaired loans		142,592	$ 2,525
Construction | Total Commercial | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	371	0
Construction | Total Commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	0
Student loans | Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 18,200	$ 22,300