Allowance for Credit Losses - Schedule of Allowance for Loan Losses and Carrying Amount of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	$ 76,653	$ 76,627	$ 72,771
Charge-offs	7,712	13,570	32,863
Recoveries	1,990	2,696	8,313
Provision	6,204	11,382	28,406
Allowance for Loan Losses Ending balance	77,135	77,135	76,627
Allowance for Loan Losses, individually evaluated for impairment	19,084	19,084	22,345
Allowance for Loan Losses, collectively evaluated for impairment	58,051	58,051	54,282
Financing receivables	13,529,008	13,529,008	13,027,337
Financing receivables, individually evaluated for impairment	163,844	163,844	168,524
Financing receivables, collectively evaluated for impairment	13,196,667	13,196,667	12,648,292
Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0	0
Financing receivables	168,497	168,497	210,521
Construction & Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	6,429	7,187	10,606
Charge-offs	72	532	3,337
Recoveries	9	11	726
Provision	226	(74)	(808)
Allowance for Loan Losses Ending balance	6,592	6,592	7,187
Allowance for Loan Losses, individually evaluated for impairment	277	277	229
Allowance for Loan Losses, collectively evaluated for impairment	6,315	6,315	6,958
Financing receivables	1,396,853	1,396,853	1,504,907
Financing receivables, individually evaluated for impairment	16,892	16,892	16,921
Financing receivables, collectively evaluated for impairment	1,357,792	1,357,792	1,461,206
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	0
Financing receivables	22,169	22,169	26,780
Allowance for Estimated Imprecision [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	170	73	347
Charge-offs	0	0	0
Recoveries	0	0	0
Provision	(13)	84	(274)
Allowance for Loan Losses Ending balance	157	157	73
Allowance for Loan Losses, individually evaluated for impairment	0	0	0
Allowance for Loan Losses, collectively evaluated for impairment	157	157	73
Financing receivables	0	0	0
Financing receivables, individually evaluated for impairment	0	0	0
Financing receivables, collectively evaluated for impairment	0	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	0
Financing receivables	0	0	0
Commercial Real Estate [Member] \| Owner-Occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	3,719	5,401	5,273
Charge-offs	728	1,743	2,246
Recoveries	690	745	2,599
Provision	(468)	(1,190)	(225)
Allowance for Loan Losses Ending balance	3,213	3,213	5,401
Allowance for Loan Losses, individually evaluated for impairment	978	978	2,251
Allowance for Loan Losses, collectively evaluated for impairment	2,235	2,235	3,150
Financing receivables	1,366,814	1,366,814	1,361,629
Financing receivables, individually evaluated for impairment	30,134	30,134	36,721
Financing receivables, collectively evaluated for impairment	1,305,807	1,305,807	1,291,379
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	0
Financing receivables	30,873	30,873	33,529
Commercial Real Estate [Member] \| Nonowner-Occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	6,774	6,369	6,883
Charge-offs	314	314	296
Recoveries	17	153	244
Provision	(294)	(25)	(462)
Allowance for Loan Losses Ending balance	6,183	6,183	6,369
Allowance for Loan Losses, individually evaluated for impairment	1,693	1,693	1,592
Allowance for Loan Losses, collectively evaluated for impairment	4,490	4,490	4,777
Financing receivables	4,435,910	4,435,910	4,451,298
Financing receivables, individually evaluated for impairment	21,653	21,653	21,851
Financing receivables, collectively evaluated for impairment	4,337,381	4,337,381	4,320,997
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	0
Financing receivables	76,876	76,876	108,450
Other Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	46,917	45,189	33,087
Charge-offs	5,795	8,663	21,189
Recoveries	984	1,090	3,395
Provision	6,085	10,575	29,896
Allowance for Loan Losses Ending balance	48,191	48,191	45,189
Allowance for Loan Losses, individually evaluated for impairment	14,676	14,676	16,721
Allowance for Loan Losses, collectively evaluated for impairment	33,515	33,515	28,468
Financing receivables	2,209,191	2,209,191	1,998,979
Financing receivables, individually evaluated for impairment	79,980	79,980	78,715
Financing receivables, collectively evaluated for impairment	2,103,205	2,103,205	1,892,706
Other Commercial [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0	0
Financing receivables	26,006	26,006	27,558
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	10,253	9,927	13,770
Charge-offs	82	992	2,973
Recoveries	94	358	601
Provision	115	1,087	(1,471)
Allowance for Loan Losses Ending balance	10,380	10,380	9,927
Allowance for Loan Losses, individually evaluated for impairment	1,460	1,460	1,552
Allowance for Loan Losses, collectively evaluated for impairment	8,920	8,920	8,375
Financing receivables	3,300,472	3,300,472	2,996,171
Financing receivables, individually evaluated for impairment	15,185	15,185	14,316
Financing receivables, collectively evaluated for impairment	3,272,737	3,272,737	2,967,666
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	0
Financing receivables	12,550	12,550	14,189
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses Beginning balance	2,391	2,481	2,805
Charge-offs	721	1,326	2,822
Recoveries	196	339	748
Provision	553	925	1,750
Allowance for Loan Losses Ending balance	2,419	2,419	2,481
Allowance for Loan Losses, individually evaluated for impairment	0	0	0
Allowance for Loan Losses, collectively evaluated for impairment	2,419	2,419	2,481
Financing receivables	819,768	819,768	714,353
Financing receivables, individually evaluated for impairment	0	0	0
Financing receivables, collectively evaluated for impairment	819,745	819,745	714,338
Consumer [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses	0	0	0
Financing receivables	$ 23	$ 23	$ 15

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

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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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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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- 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.

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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.

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