Loans Held for Investment and Allowance for Loan Losses - Additional Information (Detail)				3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Contract	Jun. 30, 2017 USD ($) Contract	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($) Contract	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 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	$ 26,685,000	$ 17,670,000	$ 21,489,000	$ 26,685,000	$ 21,489,000	$ 26,685,000	$ 21,489,000	$ 17,670,000
Contractual balance	34,412,000	23,851,000	29,931,000	34,412,000	29,931,000	34,412,000	29,931,000	23,851,000
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	28,104,000	20,117,000	24,720,000	28,104,000	24,720,000	28,104,000	24,720,000	20,117,000
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	624,000
Allowance for loan losses	49,951,000	48,156,000	47,410,000	$ 49,951,000	$ 47,410,000	49,951,000	$ 47,410,000	48,156,000	$ 49,499,000	$ 46,192,000	$ 45,779,000
Default for purposes of this disclosure is a troubled debt restructured loan				90 days	90 days
Number of loans modified | Contract				0	0		2
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	2,378,431,000			2,378,431,000		2,378,431,000
Advances from Federal Home Loan Bank of Dallas	60,000,000		50,000,000	60,000,000	$ 50,000,000	60,000,000	$ 50,000,000
Letters of credit outstanding	1,500,000,000,000			1,500,000,000,000		1,500,000,000,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans individually evaluated for impairment	2,813,000	618,000	889,000	2,813,000	889,000	2,813,000	889,000	618,000
Contractual balance	3,918,000	755,000	1,174,290	3,918,000	1,174,290	3,918,000	1,174,290	755,000
Allowance for loan losses	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0