- Fair Value Level 3 Inputs, Quantitative Information (Details) $ in Millions	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Measurement Input, Commodity Forward Price | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Energy derivative, measurement input			20.02
Measurement Input, Commodity Forward Price | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Energy derivative, measurement input			32.58
Measurement Input, Discount Rate | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Energy derivative, measurement input			0.0280
Measurement Input, Discount Rate | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Energy derivative, measurement input			0.0281
Measurement Input, Discount Rate | Broadview Project
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	0.05
Measurement Input, Discount Rate | Broadview Project | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input			0.040
Measurement Input, Discount Rate | Broadview Project | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input			0.080
Measurement Input, Discount Rate | Tsugaru Holdings | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	0.0690		0.0690
Measurement Input, Annual Energy Production Loss | Broadview Project | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input			0.0070
Measurement Input, Deferred Purchase Price | Tsugaru Holdings | Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	109		109
Measurement Input, Deferred Purchase Price | Tsugaru Holdings | Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration, measurement input	128		128
Contingent Consideration | Recurring Measurement Basis
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration	$ 122	$ 111	$ 130	$ 125	$ 128	$ 22
Contingent Consideration | Broadview Project | Recurring Measurement Basis | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration			25
Contingent Consideration | Tsugaru Holdings | Recurring Measurement Basis | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Contingent consideration	$ 122		105
Energy derivative | Recurring Measurement Basis | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Energy derivative			$ 7