Summary of Significant Accounting Policies - (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) contract
Disaggregation of Revenue [Line Items]
Number of active contracts | contract	791,000
Trust and investment services income
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 7.5
Fees on deposit accounts
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	25.8
Transferred at Point in Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	33.8
Transferred over Time
Disaggregation of Revenue [Line Items]
Revenue from contract with customers	$ 4.8