CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income (loss)	$ (173.7)	$ 433.6	$ 425.5
Pension Plan and Restoration of Retirement Income Plan:
Amortization of deferred net loss, net of tax of $1.2, $1.1 and $1.1, respectively	3.9	3.4	3.3
Net loss arising during the period, net of tax of ($1.7), ($2.6) and ($4.7), respectively	(5.1)	(8.3)	(14.1)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), after Tax	2.2	8.6	4.7
Postretirement benefit plans:
Amortization of prior service credit, net of tax of ($0.6), ($0.6) and ($0.6), respectively	(1.7)	(1.7)	(1.7)
Amortization of deferred net gain, net of tax of $0.0, $0.0 and $0.0, respectively	(0.1)	(0.2)	0.0
Net gain (loss) arising during the period, net of tax of ($0.8), ($0.1) and $0.7, respectively	(2.4)	(0.2)	2.1
Curtailment cost, net of tax of ($0.1), $0.0 and $0.0, respectively	(0.3)	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	(0.2)	(0.2)	0.0
Other comprehensive income (loss), net of tax	(4.2)	1.0	(5.7)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.7	0.6	0.0
Comprehensive income (loss)	$ (177.9)	$ 434.6	$ 419.8