Summary of Significant Accounting Policies - (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Gross interchange and debit card transaction fees	$ 11.1	$ 9.0	$ 21.7	$ 17.7
Network costs	3.0	2.3	$ 6.0	4.5
Number of active contracts | contract			803,000
Interchange and debit card transaction fees
Disaggregation of Revenue [Line Items]
Net interchange and debit card transaction fees	8.1		$ 15.7
Revenue from contract with customers	8.1		15.7
Interchange and debit card transaction fees | Before 606
Disaggregation of Revenue [Line Items]
Net interchange and debit card transaction fees		6.7		13.2
Revenue from contract with customers		$ 6.7		$ 13.2
Trust and investment services income
Disaggregation of Revenue [Line Items]
Net interchange and debit card transaction fees	7.6		15.1
Revenue from contract with customers	7.6		15.1
Fees on deposit accounts
Disaggregation of Revenue [Line Items]
Net interchange and debit card transaction fees	25.9		51.7
Revenue from contract with customers	25.9		51.7
Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Net interchange and debit card transaction fees	32.9		66.7
Revenue from contract with customers	32.9		66.7
Transferred over Time
Disaggregation of Revenue [Line Items]
Net interchange and debit card transaction fees	4.9		9.8
Revenue from contract with customers	$ 4.9		$ 9.8