CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 433.6	$ 425.5	$ 619.0
Pension Plan and Restoration of Retirement Income Plan:
Amortization of deferred net loss, net of tax of $1.1, $1.1 and $1.4, respectively	3.4	3.3	2.5
Amortization of prior service credit, net of tax of $0.0, $0.0 and $0.0, respectively	0.0	0.0	0.1
Net gain (loss) arising during the period, net of tax of $(2.6), ($4.7) and $0.2, respectively	(8.3)	(14.1)	0.4
Settlement cost, net of tax of $2.7, $1.6 and $1.4, respectively	8.6	4.7	2.2
Postretirement Benefit Plans:
Amortization of prior service credit, net of tax of ($0.6), ($0.6) and ($0.3), respectively	(1.7)	(1.7)	(0.6)
Amortization of deferred net gain, net of tax of $0.0, $0.0 and $0.0, respectively	(0.2)	0.0	0.0
Prior service cost arising during the period, net of tax of $0.0, $0.0 and $4.0, respectively	0.0	0.0	6.3
Net gain (loss) arising during the period, net of tax of ($0.1), $0.7 and ($0.2), respectively	(0.2)	2.1	(0.6)
Settlement cost, net of tax of $0.0, $0.0 and $0.2, respectively	0.0	0.0	0.5
Other comprehensive income (loss), net of tax	1.0	(5.7)	10.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.6	0.0	0.0
Comprehensive income	$ 434.6	$ 419.8	$ 629.6