Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)				3 Months Ended		12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Minimum number of days to consider the loans as non-accrual				90 days
Loans individually evaluated for impairment	$ 28,080,000	$ 27,371,000	$ 27,175,000	$ 28,080,000	$ 27,175,000	$ 27,371,000
Contractual balance	42,306,000	38,379,000	33,546,000	42,306,000	33,546,000	38,379,000
Loans held for sale	13,629,000	26,898,000	17,008,000	13,629,000	17,008,000	26,898,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	30,518,000	29,000,000	29,028,000	30,518,000	29,028,000	29,000,000
Additional funds advanced in connection with impaired loans	0			0
Interest income recognized on impaired loans	0	0	0
Interest income recognized on impaired loans				0	0	829,000
Allowance for loan losses	46,192,000	45,779,000	44,072,000	$ 46,192,000	44,072,000	45,779,000	$ 41,877,000
Default for purposes of this disclosure is a troubled debt restructured loan				90 days
Commitments to lend additional funds to borrowers with loan that have been modified as TDRs	0			$ 0
Loans held by subsidiaries subject to blanket liens	2,075,826,000			2,075,826,000
Advances from Federal Home Loan Bank of Dallas	0	75,000,000	175,000,000	0	175,000,000	75,000,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	937,000	1,256,000	1,970,000	937,000	1,970,000	1,256,000
Contractual balance	1,414,000	1,865,000	2,701,000	1,414,000	2,701,000	1,865,000
Allowance for loan losses	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0