CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Partners' Capital [Abstract]
Tax benefit (expense) on net change in postretirement benefit plans	$ 9.8	$ 2.6	$ (38.8)
Tax expense (benefit) on amortization of prior service costs	(3.8)	(2.2)	0.1
Tax (benefit) on amortization of net actuarial loss	(4.5)	(3.1)	(4.8)
Tax (expense) on prior service credit from plan amendment	0.7	(19.7)	(7.1)
Tax (expense) benefit on net actuarial loss arising during the period	8.2	23.2	(26.8)
Tax expense (benefit) on recognition of curtailment gain	$ 1.6	$ 0.0	$ 0.0