Loans Held for Investment and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 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	$ 40,444,000	$ 29,724,000	$ 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	60,440,000	51,585,000	52,499,000	$ 51,202,000
Loans individually evaluated for impairment	$ 39,226,000	$ 28,508,000	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			2,717,982,000
Advances from Federal Home Loan Bank of Dallas	446,000,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	0	$ 0
Loans individually evaluated for impairment	$ 7,773,000	$ 859,000	$ 251,000