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Common Equity - Performance Units (Details) - Performance units - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance units granted		217,560	237,650	297,305
Intrinsic value of settled performance units		$ 9.7	$ 6.7	$ 19.1
Tax benefit from distribution of performance units		$ 2.2	$ 2.1	$ 6.8
Performance units outstanding		618,822
Liability recorded on balance sheet		$ 38.1
Compensation cost not yet recognized		$ 18.4
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 3 months
Subsequent event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance units granted	148,036
Intrinsic value of settled performance units	$ 18.6
Tax benefit from distribution of performance units	$ 4.3

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

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-Name Accounting Standards Codification
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-SubTopic 10
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Amount of cash paid related to share-based compensation liabilities.

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Amount of tax benefit from the vesting or payment of awards under share-based compensation arrangements.

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