CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Partners' Capital [Abstract]
Tax benefit (expense) on net change in postretirement benefit plans	$ 2.5	$ (38.8)
Tax expense on net change in unrealized loss on investment securities			1.0
Tax expense (benefit) on amortization of prior service costs	(2.2)	0.1
Tax (benefit) on amortization of net actuarial loss	(3.1)	(4.8)
Tax (expense) on prior service credit from plan amendment	(19.7)	(7.1)
Tax (expense) benefit on net actuarial loss arising during the period	23.2	(26.8)
Tax expense on realized losses on investment securities			$ 1.0