Summary of Significant Accounting Policies - Concentrations of Credit Risk- (Details) - Concentrations of Credit Risk $ in Millions	Dec. 31, 2017 USD ($) item
Concentrations of Credit Risk
Concentration risk threshold, total risk-based capital threshold (as a percent)	25.00%
Concentration risk threshold, total risk-based capital threshold	$ 358.2
Number of credit concentrations for non-acquired credit impaired loans | item	4
Lessors of residential and buildings
Concentrations of Credit Risk
Concentration risk threshold, total risk-based capital threshold	$ 524.1
Lessors of nonresidential buildings
Concentrations of Credit Risk
Concentration risk threshold, total risk-based capital threshold	1,300.0
Loans to religious organizations
Concentrations of Credit Risk
Concentration risk threshold, total risk-based capital threshold	358.6
Loans for construction
Concentrations of Credit Risk
Concentration risk threshold, total risk-based capital threshold	$ 545.8