CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 425.5	$ 619.0	$ 338.2
Pension Plan and Restoration of Retirement Income Plan:
Amortization of deferred net loss, net of tax of $1.1, $1.4 and $1.7, respectively	3.3	2.5	2.8
Amortization of prior service cost, net of tax of $0.0, $0.0 and $0.0, respectively	0.0	(0.1)	0.0
Net gain (loss) arising during the period, net of tax of ($4.7), $0.2 and ($0.6), respectively	(14.1)	0.4	(0.7)
Settlement cost, net of tax of $1.6, $1.4 and $3.2, respectively	4.7	2.2	5.0
Postretirement Benefit Plans:
Amortization of prior service credit, net of tax of ($0.6), ($0.3) and ($1.0), respectively	(1.7)	(0.6)	(1.5)
Prior service cost arising during the period, net of tax of $0.0, $4.0 and $0.0, respectively	0.0	6.3	0.0
Net gain (loss) arising during the period, net of tax of $0.7, ($0.2) and $0.1, respectively	2.1	(0.6)	0.2
Settlement cost, net of tax of $0.0, $0.2 and $0.0, respectively	0.0	0.5	0.0
Other comprehensive income (loss), net of tax	(5.7)	10.6	5.8
Comprehensive income	$ 419.8	$ 629.6	$ 344.0