Benefit Plans, Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Amortization of prior service credit (gain) loss	$ (7.1)	$ (9.1)	$ (4.7)
Total recognized in other comprehensive income	16.1	(16.9)	$ 8.5
Pension Benefits [Member]
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain (loss)	(23.0)	6.8
Amortization of actuarial (gain) loss	7.7	10.4
Amortization of prior service credit (gain) loss	0.0	0.0
Total recognized in other comprehensive income	(15.3)	17.2
Post-Retirement Benefits [Member]
Changes in Plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial gain (loss)	(0.2)	1.0
Amortization of actuarial (gain) loss	(0.6)	(0.6)
Amortization of prior service credit (gain) loss	0.0	(0.7)
Total recognized in other comprehensive income	$ (0.8)	$ (0.3)

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- Definition

Amount, after tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, after tax, of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) of defined benefit plan.

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Amount, after tax and reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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Amount, after tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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