Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Unrealized gains (losses) arising during the period, tax expense (benefit)	$ 0.0	$ (0.2)	$ (0.6)
Reclassification adjustments for losses included in net income, tax benefit	0.7	3.4	8.4
Unrealized gains (losses) arising during the period, tax expense (benefit)	(2.0)	(3.4)	(26.7)
Amortization of net actuarial losses, tax benefit	$ 1.9	$ 1.8	$ 0.8