Investment Securities - Proceeds from Calls and Sales, Realized Gains and Losses and Interest Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Proceeds from calls and sales of investment securities
Proceeds from calls of investment securities	$ 0.0	$ 46.2	$ 0.0	$ 121.2
Proceeds from sales of investment securities	0.0	0.0	0.0	505.0
Realized investment gains and losses
Gross realized gains on sales of investments	0.0	0.0	0.0	25.8
Gross realized losses on sales of investments	0.0	0.0	0.0	0.0
Provision for income taxes related to net realized gains on sale of investment securities	0.0	0.0	0.0	10.2
Interest income from taxable and nontaxable investment securities
Taxable interest income	24.2	21.1	75.7	57.1
Nontaxable interest income	$ 0.0	$ 0.0	$ 0.0	$ 0.0