Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	May 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
Remaining long-term lease commitments		$ 18,941,000
Commitments outstanding, aggregate amount		1,305,631,000
Commitments expected to be funded in 2019		469,098,000
Short-term borrowing facility, maximum capacity		100,000,000
Short-term borrowing facility, letters of credit		55,000,000
Letters of credit outstanding, amount		$ 3,649,000	$ 2,995,000
Short-term borrowing facility, expiration date		Oct. 31, 2020
FHLB stock purchased	$ 7,000,000.0
Total amount of guarantees outstanding		$ 123,574,000
Life insurance policies with guarantees, cash value		141,759,000
Collateralized Mortgage Obligations
Financing Receivable, Impaired [Line Items]
Fair value of collateral held at FHLB		127,700,000
Commercial Mortgage Loans
Financing Receivable, Impaired [Line Items]
Fair value of collateral held at FHLB		$ 361,800,000