Allowance for Credit Losses - Schedule of Allowance for Loan Losses and Carrying Amount of Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	$ 72,771	$ 75,726
Charge-offs	32,863	36,180
Recoveries	8,313	8,716
Provision	28,406	24,509
Allowance for Loan Losses Ending balance	76,627	72,771
Allowance for Loan Losses, individually evaluated for impairment	22,345	23,417
Allowance for Loan Losses, collectively evaluated for impairment	54,282	49,354
Allowance for Loan Losses	76,627	72,771	$ 75,726	$ 75,529
Financing receivables	13,027,337	10,356,719
Financing receivables, individually evaluated for impairment	168,524	124,820
Financing receivables, collectively evaluated for impairment	12,648,292	10,060,303
Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	210,521	171,596
Construction & Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	10,606	18,205
Charge-offs	3,337	2,659
Recoveries	726	433
Provision	(808)	(5,373)
Allowance for Loan Losses Ending balance	7,187	10,606
Allowance for Loan Losses, individually evaluated for impairment	229	3,206
Allowance for Loan Losses, collectively evaluated for impairment	6,958	7,400
Financing receivables	1,504,907	1,255,738
Financing receivables, individually evaluated for impairment	16,921	8,152
Financing receivables, collectively evaluated for impairment	1,461,206	1,221,006
Construction & Land Development [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	26,780	26,580
Allowance for Estimated Imprecision [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	347	104
Charge-offs	0	0
Recoveries	0	0
Provision	(274)	243
Allowance for Loan Losses Ending balance	73	347
Allowance for Loan Losses, individually evaluated for impairment	0	0
Allowance for Loan Losses, collectively evaluated for impairment	73	347
Financing receivables	0	0
Financing receivables, individually evaluated for impairment	0	0
Financing receivables, collectively evaluated for impairment	0	0
Allowance for Estimated Imprecision [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	0	0
Commercial Real Estate [Member] \| Owner-Occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	5,273	3,637
Charge-offs	2,246	5,281
Recoveries	2,599	3,071
Provision	(225)	3,846
Allowance for Loan Losses Ending balance	5,401	5,273
Allowance for Loan Losses, individually evaluated for impairment	2,251	815
Allowance for Loan Losses, collectively evaluated for impairment	3,150	4,458
Financing receivables	1,361,629	1,049,885
Financing receivables, individually evaluated for impairment	36,721	18,976
Financing receivables, collectively evaluated for impairment	1,291,379	1,005,999
Commercial Real Estate [Member] \| Owner-Occupied [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	33,529	24,910
Commercial Real Estate [Member] \| Nonowner-Occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	6,883	5,309
Charge-offs	296	419
Recoveries	244	675
Provision	(462)	1,318
Allowance for Loan Losses Ending balance	6,369	6,883
Allowance for Loan Losses, individually evaluated for impairment	1,592	2,524
Allowance for Loan Losses, collectively evaluated for impairment	4,777	4,359
Financing receivables	4,451,298	3,425,453
Financing receivables, individually evaluated for impairment	21,851	26,835
Financing receivables, collectively evaluated for impairment	4,320,997	3,323,117
Commercial Real Estate [Member] \| Nonowner-Occupied [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	108,450	75,501
Other Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	33,087	31,328
Charge-offs	21,189	20,430
Recoveries	3,395	3,452
Provision	29,896	18,737
Allowance for Loan Losses Ending balance	45,189	33,087
Allowance for Loan Losses, individually evaluated for impairment	16,721	13,441
Allowance for Loan Losses, collectively evaluated for impairment	28,468	19,646
Financing receivables	1,998,979	1,613,437
Financing receivables, individually evaluated for impairment	78,715	56,091
Financing receivables, collectively evaluated for impairment	1,892,706	1,527,479
Other Commercial [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	27,558	29,867
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	13,770	15,148
Charge-offs	2,973	4,597
Recoveries	601	639
Provision	(1,471)	2,580
Allowance for Loan Losses Ending balance	9,927	13,770
Allowance for Loan Losses, individually evaluated for impairment	1,552	3,431
Allowance for Loan Losses, collectively evaluated for impairment	8,375	10,339
Financing receivables	2,996,171	2,403,437
Financing receivables, individually evaluated for impairment	14,316	14,766
Financing receivables, collectively evaluated for impairment	2,967,666	2,373,969
Residential Real Estate [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	14,189	14,702
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	2,805	1,995
Charge-offs	2,822	2,794
Recoveries	748	446
Provision	1,750	3,158
Allowance for Loan Losses Ending balance	2,481	2,805
Allowance for Loan Losses, individually evaluated for impairment	0	0
Allowance for Loan Losses, collectively evaluated for impairment	2,481	2,805
Financing receivables	714,353	608,769
Financing receivables, individually evaluated for impairment	0	0
Financing receivables, collectively evaluated for impairment	714,338	608,733
Consumer [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0
Financing receivables	$ 15	$ 36

Amount of expense (or recovery of expense) related to the estimated loss from loan and lease transactions.

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Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

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- Definition

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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