Loans Held for Investment and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Jun. 30, 2019		Dec. 31, 2018		Jul. 30, 2019	Mar. 31, 2019	Jun. 30, 2018		Mar. 31, 2018	Dec. 31, 2017
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		$ 29,632,000				$ 28,104,000
Non accrual loans	26,408,000	[1]	27,534,000	[1]	$ 26,408,000		26,685,000	[1]
Additional funds advanced in connection with impaired loans	0
Interest income recognized on impaired loans			948,000
Allowance for loan losses	51,820,000		51,202,000			$ 51,585,000	49,951,000		$ 49,499,000	$ 48,156,000
Loans individually evaluated for impairment	$ 26,408,000		27,534,000				26,685,000
Default for purposes of this disclosure is a troubled debt restructured loan	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	2,563,225,000
Impaired Financing Receivable, Unpaid Principal Balance	35,235,000		36,614,000				34,412,000
Purchased Credit Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non accrual loans	464,000		827,000				2,813,000
Allowance for loan losses	0		0				0
Loans individually evaluated for impairment	464,000		827,000				2,813,000
Impaired Financing Receivable, Unpaid Principal Balance	$ 750,000		$ 1,157,000				$ 3,918,000

[1] Includes $464,000, $2,813,000 and $827,000 of purchased credit impaired loans as of June 30, 2019 and 2018, and December 31, 2018, respectively.