Allowance for Loan and Lease Losses and Credit Quality Information - Summary of Nonaccrual and Past Due Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 48,833	$ 13,343
Accruing Current Balances	8,365,872	4,835,916
Acquired Impaired Loans	26,935	14,944	$ 21,654
Nonaccrual Loans	22,922	30,054
Ending balance	$ 8,464,562	$ 4,894,257	4,805,094
30-89 Days Past Due and Still Accruing, % of Total Loans	0.39%	0.25%
Greater Than 90 Days Past Due and Still Accruing, % of Total Loans	0.19%	0.02%
Total Past Due And Still Accruing, % of Total Loans	0.58%	0.27%
Accruing Current Balances, % of Total Loans	98.83%	98.81%
Acquired Impaired Loans, % of Total Loans	0.32%	0.31%
Nonaccrual Loans, % of Total Loans	0.27%	0.61%
% of Total Loans	100.00%	100.00%
Reverse mortgage, fair value	$ 16,600	$ 16,500	19,800
Acquired nonimpaired loans	3,221,316	429,982	565,516
30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	32,683	12,508
Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	16,150	835
Owner-occupied commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	2,329	1,598
Accruing Current Balances	1,283,320	1,049,722
Acquired Impaired Loans	6,757	4,248	5,803
Nonaccrual Loans	4,060	4,406
Ending balance	1,296,466	1,059,974	1,079,247
Acquired nonimpaired loans	313,955	112,386	136,437
Owner-occupied commercial | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	1,498	1,598
Owner-occupied commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	831	0
Commercial mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	5,098	2,296
Accruing Current Balances	2,207,582	1,148,988
Acquired Impaired Loans	8,670	7,504	9,724
Nonaccrual Loans	1,626	3,951
Ending balance	2,222,976	1,162,739	1,187,705
Acquired nonimpaired loans	1,107,379	145,648	188,505
Commercial mortgages | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	4,999	2,296
Commercial mortgages | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	99	0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	157
Accruing Current Balances	580,591	312,879
Acquired Impaired Loans	491	749	940
Nonaccrual Loans	0	2,781
Ending balance	581,082	316,566	281,608
Acquired nonimpaired loans	142,592	2,525	15,759
Construction | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	157
Construction | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	0	0
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	8,327	4,717
Accruing Current Balances	2,214,506	1,452,185
Acquired Impaired Loans	1,564	1,531	4,156
Nonaccrual Loans	11,031	14,056
Ending balance	2,235,428	1,472,489	1,464,554
Acquired nonimpaired loans	603,157	89,970	116,566
Commercial | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	6,289	4,646
Commercial | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	2,038	71
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	7,170	2,975
Accruing Current Balances	980,893	194,960
Acquired Impaired Loans	7,326	761	784
Nonaccrual Loans	4,490	2,854
Ending balance	999,879	201,550	233,487
Acquired nonimpaired loans	834,820	57,708	72,304
Residential | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	6,733	2,315
Residential | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	437	660
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	25,909	1,600
Accruing Current Balances	1,098,980	677,182
Acquired Impaired Loans	2,127	151	247
Nonaccrual Loans	1,715	2,006
Ending balance	1,128,731	680,939	558,493
Acquired nonimpaired loans	219,413	21,745	$ 35,945
Consumer | 30–89 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	13,164	1,496
Consumer | Greater Than 90 Days Past Due and Still Accruing
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	12,745	$ 104
Consumer | Student loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due And Still Accruing	$ 22,300