Loans Held for Investment and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual, past due 90 days or more and still accruing, restructured loans and foreclosed assets	$ 27,860,000	$ 39,724,000	$ 25,770,000
Additional funds advanced in connection with impaired loans		0
Interest income recognized on impaired loans		0
Interest income recognized on impaired loans			750,000
Allowance for loan losses	51,820,000	68,947,000	52,499,000	$ 60,440,000	$ 51,585,000	$ 51,202,000
Loans individually evaluated for impairment	$ 26,408,000	$ 39,320,000	24,582,000
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
Loans held by subsidiaries subject to blanket liens		3,134,584,000
Advances from Federal Home Loan Bank of Dallas		0
loans subject to deferral and modification agreements	$ 4,040,952,000	5,253,067,000	4,194,969,000
Deferral And Modification Agreement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
loans subject to deferral and modification agreements		$ 468,539,000,000,000
Outstanding loans held for investment Percentage		10.30%
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	0	$ 0
Loans individually evaluated for impairment	$ 464,000	$ 7,275,000	$ 251,000