Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unrealized gains and immaterial losses on Master Trust assets in AOCI, after tax		$ 0.0	$ 0.0	$ 0.4
Asset Retirement Obligation, Revision of Estimate		(3.4)	2.6
Proceeds from Sale of Debt Securities, Available-for-sale		0.3
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax		$ 0.2
AOCI reclassed to Retained Earnings before tax	$ 1.6
AOCI reclassed to Retained Earnings, net of tax	$ 1.0		$ (1.0)

AOCI reclassed to Retained Earnings before tax

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- Definition

AOCI reclassed to Retained Earnings, net of tax

+ References

No definition available.

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- Definition

Realized gains and losses from sale of available for sale securities, net of tax.

+ References

No definition available.

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- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

+ References

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- Definition

Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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