Loans and Allowance for Loan Losses - Additional Information (Detail)				3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($) Contract	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($) Contract	Dec. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
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	$ 18,750,000	$ 27,371,000	$ 33,712,000	$ 18,750,000	$ 33,712,000	$ 18,750,000	$ 33,712,000	$ 27,371,000
Contractual balance	29,749,000	38,379,000	47,473,000	29,749,000	47,473,000	29,749,000	47,473,000	38,379,000
Loans held for sale	19,119,000	26,898,000	31,591,000	19,119,000	31,591,000	19,119,000	31,591,000	26,898,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	22,076,000	29,000,000	34,938,000	22,076,000	34,938,000	22,076,000	34,938,000	29,000,000
Additional funds advanced in connection with impaired loans	0			0		0
Interest income recognized on impaired loans	0	0	0
Interest income recognized on impaired loans				0	0	0	0	829,000
Allowance for loan losses	47,922,000	45,779,000	45,298,000	$ 47,922,000	$ 45,298,000	$ 47,922,000	$ 45,298,000	45,779,000	$ 47,410,000	$ 45,060,000	$ 41,877,000
Default for purposes of this disclosure is a troubled debt restructured loan						90 days	90 days
Number of loans modified | Contract				2	2	4	3
Commitments to lend additional funds to borrowers with loan that have been modified as TDRs	0			$ 0		$ 0
Loans held by subsidiaries subject to blanket liens	2,140,557,000			2,140,557,000		2,140,557,000
Advances from Federal Home Loan Bank of Dallas	0	75,000,000	160,000,000	0	$ 160,000,000	0	$ 160,000,000	75,000,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	736,000	1,256,000	1,853,000	736,000	1,853,000	736,000	1,853,000	1,256,000
Contractual balance	932,000	1,865,000	2,528,000	932,000	2,528,000	932,000	2,528,000	1,865,000
Allowance for loan losses	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0