The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Nominal number of units used to calculate payment on derivative.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Derivative Instruments - Additional Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) MW	Dec. 31, 2018 MW	Dec. 31, 2010 MW
Derivative [Line Items]
AOCL to be reclassified within 12 months \| $	$ 6
Not Designated as Hedging Instrument \| Energy derivative
Derivative [Line Items]
Nonmonetary notional amount \| MW	0	193,252	504,220
Minimum \| Designated Derivative Instruments \| Interest rate swaps
Derivative [Line Items]
Remaining maturities	4 years 2 months 12 days
Minimum \| Not Designated as Hedging Instrument \| Foreign currency forward contracts
Derivative [Line Items]
Remaining maturities	3 months
Maximum \| Designated Derivative Instruments \| Interest rate swaps
Derivative [Line Items]
Remaining maturities	23 years 6 months
Maximum \| Not Designated as Hedging Instrument \| Foreign currency forward contracts
Derivative [Line Items]
Remaining maturities	10 years 6 months