CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit), Tax	$ 0.3	$ 0.0	$ 0.0
Net actuarial gain (loss) arising during period, tax	(6.0)	(10.6)	20.3
Other Comprehensive Income (Loss), Defined Benefit Plans, Settlement Effects Arising During Period, Tax	18.7	0.0	0.0
Less: amortization of actuarial loss, tax	1.6	1.1	3.6
Less: amortization of prior service credit, tax	(0.5)	(0.5)	(0.5)
Net gains on investment securities, tax	0.4	0.2	2.1
Net gains (losses) on derivatives, tax	$ 0.8	$ 0.9	$ 1.8