Borrowings - Additional Information (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate	$ 10,900,000
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	624,000,000	$ 675,000,000
Short-term borrowings	610,000,000
Long-term borrowings	14,000,000
Maximum borrowing capacity	2,300,000,000.0
Remaining borrowing capacity	1,500,000,000
Borrowing capacity utilized	800,000,000
Letter of credit to collateralize public funds	176,000,000
Mortgage Backed Securities
Long Term And Short Term Debt [Line Items]
Mortgage backed securities pledged as collateral	$ 49,000,000	$ 53,200,000