Borrowings - Additional Information (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate	$ 8,500,000
Total long-term debt outstanding at a variable rate	38,100,000
Federal Home Loan Bank Advance
Long Term And Short Term Debt [Line Items]
Total borrowings	572,000,000	$ 587,000,000
Short-term borrowings	525,000,000
Long-term borrowings	46,700,000
Maximum borrowing capacity	2,500,000,000.0
Borrowing capacity utilized	709,000,000
Letter of credit to collateralize public funds	137,000,000
Remaining borrowing capacity	1,800,000,000
Mortgage Backed Securities
Long Term And Short Term Debt [Line Items]
Mortgage backed securities pledged as collateral	$ 49,200,000	$ 56,800,000