CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Partners' Capital [Abstract]
Tax benefit (expense) on net change in postretirement benefit plans	$ (6.9)	$ 9.8	$ 2.6
Tax benefit on net change in unrealized loss on investment securities	0.1
Tax expense (benefit) on amortization of prior service costs	(2.1)	(3.8)	(2.2)
Tax (benefit) on amortization of net actuarial loss	(4.4)	(4.5)	(3.1)
Tax expense (benefit) on recognition of curtailment gain	0.0	1.6	0.0
Tax expense (benefit) on recognition of plan settlement costs	(4.7)	0.0	0.0
Tax (expense) on prior service credit from plan amendment		0.7	(19.7)
Tax (expense) benefit on net actuarial loss arising during the period	0.1	$ 8.2	$ 23.2
Tax benefit on realized losses on investment securities	$ (0.1)