Loans Held for Investment and Allowance for Loan Losses - Additional Information (Detail) - USD ($)			12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	$ 29,632,000	$ 20,117,000	$ 29,632,000	$ 20,117,000
Non accrual loans	27,534,000	17,670,000	27,534,000	17,670,000
Additional funds advanced in connection with impaired loans	0		0
Interest income recognized on impaired loans	0	0
Interest income recognized on impaired loans			948,000	624,000	$ 829,000
Allowance for loan losses	51,202,000	48,156,000	51,202,000	48,156,000	$ 45,779,000
Loans individually evaluated for impairment	27,534,000	17,670,000	$ 27,534,000	$ 17,670,000
Default for purposes of this disclosure is a troubled debt restructured loan			90 days	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	2,495,150,000		2,495,150,000
Letters of credit outstanding	55,000,000		55,000,000
Advances from Federal Home Loan Bank of Dallas	55,000,000	0	55,000,000	$ 0
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans	827,000	618,000	827,000	618,000
Allowance for loan losses	0	0	0	0
Loans individually evaluated for impairment	$ 827,000	$ 618,000	$ 827,000	$ 618,000