CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 183.4	$ 183.6	$ 324.2	$ 280.3
Pension Plan and Restoration of Retirement Income Plan:
Amortization of deferred net loss, net of tax of $0.5, $0.4, $1.3 and $1.2, respectively	0.6	0.8	2.0	2.3
Settlement cost, net of tax of $0.0, $0.0, $0.0 and $3.2, respectively	0.0	0.0	0.0	5.0
Postretirement Benefit Plans:
Prior service cost arising during the period, net of tax of $4.1, $0.0, $4.1 and $0.0, respectively	6.7	0.0	6.7	0.0
Amortization of prior service cost, net of tax of ($0.2), ($0.2), ($0.2) and ($0.7), respectively	(0.2)	(0.4)	(0.2)	(1.2)
Settlement cost, net of tax of $0.2, $0.0, $0.2 and $0.0, respectively	0.5	0.0	0.5	0.0
Other comprehensive income, net of tax	7.6	0.4	9.0	6.1
Comprehensive income	$ 191.0	$ 184.0	$ 333.2	$ 286.4