CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME Parenthetical - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 1.1	$ 1.1	$ 1.4
Net gain (loss) arising during the period, net of tax of $(2.6), ($4.7) and $0.2, respectively	(2.6)	(4.7)	0.2
Amortization of prior service credit, net of tax of $0.0, $0.0 and $0.0, respectively	0.0	0.0	0.0
Postretirement plans:
Prior service cost arising during the period, net of tax of $0.0, $0.0 and $4.0, respectively	0.0	0.0	4.0
Amortization of deferred net gain, net of tax of $0.0, $0.0 and $0.0, respectively	0.0	0.0	0.0
Net gain (loss) arising during the period, net of tax of ($0.1), $0.7 and ($0.2), respectively	(0.1)	0.7	(0.2)
Amortization of prior service credit, net of tax of ($0.6), ($0.6) and ($0.3), respectively	(0.6)	(0.6)	(0.3)
Amortization of deferred interest rate swap hedging losses, net of tax of $0, $0 and $0.0, respectively	0.0	0.0	0.0
Pension Plans [Member]
Pension Plan and Restoration of Retirement Income Plan:
Settlement cost, net of tax of $2.7, $1.6 and $1.4, respectively	2.7	1.6	1.4
Other Postretirement Benefits Plan [Member]
Pension Plan and Restoration of Retirement Income Plan:
Settlement cost, net of tax of $2.7, $1.6 and $1.4, respectively	$ 0.0	$ 0.0
Postretirement plans:
Settlement cost, net of tax of $0.0, $0.0 and $0.2, respectively			$ 0.2