CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Prior service credit arising during period, tax	$ 0.0	$ 1.1	$ 0.3
Net actuarial gain (loss) arising during period, tax	1.3	(4.8)	(6.0)
Settlement effects arising during period, tax	0.0	1.1	18.7
Less: amortization of actuarial loss, tax	0.5	1.2	1.6
Less: amortization of prior service credit, tax	(0.5)	(0.5)	(0.5)
Net gains on investment securities, tax	(2.5)	(0.1)	0.4
Net gains (losses) on derivatives, tax	$ (0.1)	$ 0.1	$ 0.8