CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Partners' Capital [Abstract]
Tax benefit (expense) on net change in postretirement benefit plans	$ (18.7)	$ 0.2	$ (1.6)
Tax expense (benefit) on amortization of prior service benefit	(1.2)	(1.0)	(1.1)
Tax (benefit) on amortization of net actuarial loss	(1.8)	(1.8)	(2.4)
Tax expense (benefit) on recognition of plan settlement costs	(25.7)	0.0	(0.4)
Tax benefit (expense) on prior service credit from plan amendment	0.0	0.4	(0.1)
Tax benefit (expense) benefit on net actuarial loss arising during the period	$ 7.6	$ 0.6	$ 0.2