Fair Value Information - Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets
Total cash and cash equivalents	$ 194,475	$ 144,377	$ 143,790	$ 121,633
Interest-earning balances due from depository institutions	8,812	17,952
FHLB stock	17,688	17,688
Investment securities available-for-sale	1,806,231	2,080,985
Investment securities held-to-maturity	759,029	829,890
Total loans, net of allowance for loan losses	7,522,452	4,771,046
Deposits:
Noninterest-bearing	5,224,154	3,846,436
Interest-bearing	3,885,672	2,700,417
Junior subordinated debentures	25,774	25,774
Swaps	400,421	556,984
Carrying Value [Member]
Assets
Total cash and cash equivalents	194,475	144,377
Interest-earning balances due from depository institutions	8,812	17,952
FHLB stock		17,688
Investment securities available-for-sale	1,806,231	2,080,985
Investment securities held-to-maturity	759,029	829,890
Total loans, net of allowance for loan losses	7,522,452	4,771,046
Swaps	944	3,211
Deposits:
Noninterest-bearing		3,846,436
Interest-bearing	3,885,672	2,700,417
Borrowings	429,477	553,773
Junior subordinated debentures	25,774	25,774
Swaps	944	3,211
Estimated Fair Value [Member]
Assets
Total cash and cash equivalents	194,475	144,377
Interest-earning balances due from depository institutions	8,723	17,951
FHLB stock		17,688
Investment securities available-for-sale	1,806,231	2,080,985
Investment securities held-to-maturity	726,755	819,215
Total loans, net of allowance for loan losses	7,218,542	4,678,402
Swaps	944	3,211
Deposits:
Noninterest-bearing		3,846,436
Interest-bearing	3,875,171	2,697,781
Borrowings	429,092	553,416
Junior subordinated debentures	21,218	18,070
Swaps	944	3,211
Estimated Fair Value [Member] \| Level 1 [Member]
Assets
Total cash and cash equivalents	194,475	144,377
Interest-earning balances due from depository institutions	0	0
FHLB stock		0
Investment securities available-for-sale	0	0
Investment securities held-to-maturity	0	0
Total loans, net of allowance for loan losses	0	0
Swaps	0	0
Deposits:
Noninterest-bearing		3,846,436
Interest-bearing	0	0
Borrowings	0	0
Junior subordinated debentures	0	0
Swaps	0	0
Estimated Fair Value [Member] \| Level 2 [Member]
Assets
Total cash and cash equivalents	0	0
Interest-earning balances due from depository institutions	8,723	17,951
FHLB stock		17,688
Investment securities available-for-sale	1,806,231	2,080,985
Investment securities held-to-maturity	726,755	819,215
Total loans, net of allowance for loan losses	0	0
Swaps	944	3,211
Deposits:
Noninterest-bearing		0
Interest-bearing	3,875,171	2,697,781
Borrowings	429,092	553,416
Junior subordinated debentures	0	0
Swaps	944	3,211
Estimated Fair Value [Member] \| Level 3 [Member]
Assets
Total cash and cash equivalents	0	0
Interest-earning balances due from depository institutions	0	0
FHLB stock		0
Investment securities available-for-sale	0	0
Investment securities held-to-maturity	0	0
Total loans, net of allowance for loan losses	7,218,542	4,678,402
Swaps	0	0
Deposits:
Noninterest-bearing		0
Interest-bearing	0	0
Borrowings	0	0
Junior subordinated debentures	21,218	18,070
Swaps	$ 0	$ 0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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