Investment Securities - Proceeds from Calls and Sales, Realized Gains and Losses and Interest Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Proceeds from calls and sales of investment securities
Proceeds from calls of investment securities	$ 121.2	$ 25.0
Proceeds from sales of investment securities	825.4	2,500.0	$ 61.9
Realized investment gains and losses
Gross realized gains on sales of investments	27.4	18.8	20.8
Gross realized losses on sales of investments	0.1	6.5	0.0
Income tax expense related to net realized gains on the sale of investment securities	10.8	4.9	8.2
Interest income from taxable and nontaxable investment securities
Taxable interest income	83.0	73.6	64.1
Nontaxable interest income	$ 0.0	$ 0.0	$ 0.0