Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 0.0	$ 1.1	$ 0.4
Gross realized losses	0.1	$ 0.4	$ 0.1
State, county and municipal securities
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	115.5
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	$ 66.8