Borrowings - Additional Information (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate	$ 9,700,000
Total long-term debt outstanding at a variable rate	3,100,000
Capital lease	100,000
Federal Home Loan Bank Advance
Long Term And Short Term Debt [Line Items]
Total borrowings	698,000,000	$ 675,000,000
Short-term borrowings	685,000,000
Long-term borrowings	12,900,000
Maximum borrowing capacity	2,500,000,000.0
Borrowing capacity utilized	902,000,000
Letter of credit to collateralize public funds	204,000,000
Remaining borrowing capacity	1,600,000,000
Mortgage Backed Securities
Long Term And Short Term Debt [Line Items]
Mortgage backed securities pledged as collateral	$ 46,700,000	$ 53,200,000