Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Conversion of Stock [Line Items]
Reserves to satisfy Federal regulatory requirements	$ 136,370,000	$ 119,317,000
Rent expense under operating leases		6,348,000	$ 5,885,000
Rent expense offset by rent income		$ 42,000	$ 44,000
Commitment expiration date	one year or less or other termination clauses and may require payment of a fee
Letters of credit, issue period	one year or less
Overdraft privilege receivable criteria	all holders of such accounts who bring their accounts to a positive balance at least once every thirty days receive the overdraft privilege
Common stock, shares outstanding (in shares) | shares	30,523,824	30,417,223
Conversion ratio	1.6228
Common Class B
Conversion of Stock [Line Items]
Common stock, shares outstanding (in shares) | shares	13,396
Common Class A
Conversion of Stock [Line Items]
The value of the Class A shares | $ / shares	$ 187.90
Value of unredeemed Class A equivalent shares	$ 4,085,000