Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2020
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions. These estimates and assumptions impact the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could be different from these estimates.
Revenue Recognition	Revenue Recognition Revenue from Contracts with Customers The Company applies the guidance in ASC 606, Revenue from Contracts with Customers, or Topic 606, when recognizing revenue associated with its contracts with customers. The Company's policies with respect to its various revenue streams are detailed below. In general, the Company applies the invoicing practical expedient to recognize revenue for the revenue streams detailed below, except in circumstances where the invoiced amount does not represent the value transferred to the customer. Thermal Revenues Steam and chilled water revenue is recognized as the Company transfers the product to the customer, based on customer usage as determined by meter readings taken at month-end. Some locations read customer meters throughout the month and recognize estimated revenue for the period between meter read date and month-end. For thermal contracts, the Company’s performance obligation to deliver steam and chilled water is satisfied over time and revenue is recognized based on the invoiced amount. The Thermal Business subsidiaries collect, and remit state and local taxes associated with sales to their customers, as required by governmental authorities. These taxes are presented on a net basis in the income statement. As contracts for steam and chilled water are long-term contracts, the Company has performance obligations under these contracts that have not yet been satisfied. These performance obligations have transaction prices that are both fixed and variable, and which vary based on the contract duration, customer type, inception date and other contract-specific factors. For the fixed price contracts, the Company cannot accurately estimate the amount of its unsatisfied performance obligations as it will vary based on customer usage, which will depend on factors such as weather and customer activity. Power Purchase Agreements The majority of the Company’s revenues are obtained through PPAs or other contractual agreements. Energy, capacity and where applicable, renewable attributes, from the majority of the Company’s renewable energy assets and certain conventional energy plants is sold through long-term PPAs and tolling agreements to a single counterparty, which is often a utility or commercial customer. The majority of these PPAs are accounted for as leases. Previously ASC 840, and currently ASC 842, requires the minimum lease payments received to be amortized over the term of the lease and contingent rentals are recorded when the achievement of the contingency becomes probable. Management's judgment is required in determining the economic life of each generating facility, in evaluating whether certain lease provisions constitute minimum payments or represent contingent rent and other factors in determining whether a contract contains a lease and whether the lease is an operating lease or sales-type lease. Renewable Energy Credits Renewable energy credits, or RECs, are usually sold through long-term PPAs. Revenue from the sale of self-generated RECs is recognized when the related energy is generated and simultaneously delivered even in cases where there is a certification lag as it has been deemed to be perfunctory. In a bundled contract to sell energy, capacity and/or self-generated RECs, all performance obligations are deemed to be delivered at the same time and hence, timing of recognition of revenue for all performance obligations is the same and occurs over time. In such cases, it is often unnecessary to allocate transaction price to multiple performance obligations. Contract Amortization Assets and liabilities recognized from power sales agreements assumed through acquisitions related to the sale of electric capacity and energy in future periods for which the fair value has been determined to be significantly less (more) than market are amortized to revenue over the term of each underlying contract based on actual generation and/or contracted volumes or on a straight-line basis, where applicable.
Recently Issued Accounting Standards Not Yet Adopted	Recently Issued Accounting Standards Adopted in 2020 In March 2020, the FASB issued ASU 2020-4, Facilitation of the Effects of Reference Rate Reform on Financial Reporting. The amendments provide for optional expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform if certain criteria is met. These amendments apply only to contracts that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. The guidance is effective for all entities as of March 12, 2020 through December 31, 2022. The Company intends to apply the amendments to all its eligible contract modifications where applicable. Recently Issued Accounting Standards Not Yet Adopted In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The amendments in this ASU simplify the accounting for income taxes by removing certain exceptions to the general principles in Topic 740, Income Taxes. The amendments also improve consistent application of and simplify GAAP for other areas of Topic 740 by clarifying and amending existing guidance. The guidance is effective January 1, 2021, with early adoption permitted. The Company does not expect the effect of the new guidance to be material on its consolidated financial statements.
Reclassification	Reclassification Certain prior year amounts have been reclassified for comparative purposes.
Segment Reporting	The Company’s segment structure reflects how management currently operates and allocates resources. The Company's businesses are segregated based on conventional power generation, renewable businesses which consist of solar and wind, and the thermal and chilled water business. The Corporate segment reflects the Company's corporate costs. The Company's chief operating decision maker, its Chief Executive Officer, evaluates the performance of its segments based on operational measures including adjusted earnings before interest, taxes, depreciation and amortization, or Adjusted EBITDA, and CAFD, as well as economic gross margin and net income (loss).

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2020

The preparation of consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions. These estimates and assumptions impact the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could be different from these estimates.

Revenue Recognition

Revenue from Contracts with Customers

The Company applies the guidance in ASC 606, Revenue from Contracts with Customers, or Topic 606, when recognizing revenue associated with its contracts with customers. The Company's policies with respect to its various revenue streams are detailed below. In general, the Company applies the invoicing practical expedient to recognize revenue for the revenue streams detailed below, except in circumstances where the invoiced amount does not represent the value transferred to the customer.

Thermal Revenues

Steam and chilled water revenue is recognized as the Company transfers the product to the customer, based on customer usage as determined by meter readings taken at month-end. Some locations read customer meters throughout the month and recognize estimated revenue for the period between meter read date and month-end. For thermal contracts, the Company’s performance obligation to deliver steam and chilled water is satisfied over time and revenue is recognized based on the invoiced amount. The Thermal Business subsidiaries collect, and remit state and local taxes associated with sales to their customers, as required by governmental authorities. These taxes are presented on a net basis in the income statement.

As contracts for steam and chilled water are long-term contracts, the Company has performance obligations under these contracts that have not yet been satisfied. These performance obligations have transaction prices that are both fixed and variable, and which vary based on the contract duration, customer type, inception date and other contract-specific factors. For the fixed price contracts, the Company cannot accurately estimate the amount of its unsatisfied performance obligations as it will vary based on customer usage, which will depend on factors such as weather and customer activity.

Power Purchase Agreements

The majority of the Company’s revenues are obtained through PPAs or other contractual agreements. Energy, capacity and where applicable, renewable attributes, from the majority of the Company’s renewable energy assets and certain conventional energy plants is sold through long-term PPAs and tolling agreements to a single counterparty, which is often a utility or commercial customer. The majority of these PPAs are accounted for as leases. Previously ASC 840, and currently ASC 842, requires the minimum lease payments received to be amortized over the term of the lease and contingent rentals are recorded when the achievement of the contingency becomes probable. Management's judgment is required in determining the economic life of each generating facility, in evaluating whether certain lease provisions constitute minimum payments or represent contingent rent and other factors in determining whether a contract contains a lease and whether the lease is an operating lease or sales-type lease.

Renewable Energy Credits

Renewable energy credits, or RECs, are usually sold through long-term PPAs. Revenue from the sale of self-generated RECs is recognized when the related energy is generated and simultaneously delivered even in cases where there is a certification lag as it has been deemed to be perfunctory.

In a bundled contract to sell energy, capacity and/or self-generated RECs, all performance obligations are deemed to be delivered at the same time and hence, timing of recognition of revenue for all performance obligations is the same and occurs over time. In such cases, it is often unnecessary to allocate transaction price to multiple performance obligations.

Contract Amortization

Assets and liabilities recognized from power sales agreements assumed through acquisitions related to the sale of electric capacity and energy in future periods for which the fair value has been determined to be significantly less (more) than market are amortized to revenue over the term of each underlying contract based on actual generation and/or contracted volumes or on a straight-line basis, where applicable.

Recently Issued Accounting Standards Not Yet Adopted

Recently Issued Accounting Standards Adopted in 2020

In March 2020, the FASB issued ASU 2020-4, Facilitation of the Effects of Reference Rate Reform on Financial Reporting. The amendments provide for optional expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform if certain criteria is met. These amendments apply only to contracts that reference LIBOR or another reference rate expected to be discontinued because of reference rate reform. The guidance is effective for all entities as of March 12, 2020 through December 31, 2022. The Company intends to apply the amendments to all its eligible contract modifications where applicable.

Recently Issued Accounting Standards Not Yet Adopted

In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The amendments in this ASU simplify the accounting for income taxes by removing certain exceptions to the general principles in Topic 740, Income Taxes. The amendments also improve consistent application of and simplify GAAP for other areas of Topic 740 by clarifying and amending existing guidance. The guidance is effective January 1, 2021, with early adoption permitted. The Company does not expect the effect of the new guidance to be material on its consolidated financial statements.

Reclassification

Certain prior year amounts have been reclassified for comparative purposes.

Segment Reporting

The Company’s segment structure reflects how management currently operates and allocates resources. The Company's businesses are segregated based on conventional power generation, renewable businesses which consist of solar and wind, and the thermal and chilled water business. The Corporate segment reflects the Company's corporate costs. The Company's chief operating decision maker, its Chief Executive Officer, evaluates the performance of its segments based on operational measures including adjusted earnings before interest, taxes, depreciation and amortization, or Adjusted EBITDA, and CAFD, as well as economic gross margin and net income (loss).