Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Undisbursed loans and unused lines of credit
Financial Instruments with Off-Balance Sheet Risk
Allowance for credit losses	$ 1.9	$ 1.1
Commitments to extend credit
Financial Instruments with Off-Balance Sheet Risk
Financial instruments with off-balance sheet risk	1,200.0	$ 581.0
Undisbursed Commitments For C&I Loans
Financial Instruments with Off-Balance Sheet Risk
Financial instruments with off-balance sheet risk	$ 707.0