Debt - Additional Information (Details) - USD ($)			1 Months Ended
	Jul. 31, 2019	Nov. 21, 2017	Sep. 30, 2019	Aug. 31, 2018	Mar. 31, 2018	Dec. 31, 2018	Jun. 30, 2018	Jul. 31, 2015
Line of Credit Facility [Line Items]
Long-term debt, gross			$ 2,590,000,000			$ 2,315,000,000
Convertible debt
Line of Credit Facility [Line Items]
Principal			$ 225,000,000			225,000,000		$ 225,000,000
Contractual interest rate								4.00%
Gulf Wind Promissory Note
Line of Credit Facility [Line Items]
Contractual interest rate			2.84%
Long-term debt, gross			$ 22,000,000			0
Tsugaru Construction Loan
Line of Credit Facility [Line Items]
Contractual interest rate			0.72%
Long-term debt, gross			$ 276,000,000			131,000,000
Tsugaru Construction Loan | Tokyo Interbank Offered Rate (TIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate					0.65%
Japanese Consumption Tax Facility
Line of Credit Facility [Line Items]
Commitment fee percentage					0.30%
Tsugaru Holdings Loan Agreement
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 70,000,000
Contractual interest rate			3.13%
Commitment fee percentage					0.50%
Long-term debt, gross			$ 62,000,000			59,000,000
Tsugaru Holdings Loan Agreement | Tokyo Interbank Offered Rate (TIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate					3.00%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 440,000,000	440,000,000
Debt term		5 years
Remaining borrowing capacity			347,000,000
Letters of credit issued			54,000,000			$ 45,000,000
Incremental Bank Loan
Line of Credit Facility [Line Items]
Debt term	3 years
Line of credit term loan	$ 250,000,000
Incremental Bank Loan | Federal Funds Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Incremental Bank Loan | Eurodollar
Line of Credit Facility [Line Items]
Interest rate	1.00%
Incremental Bank Loan | Minimum | Eurodollar
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.175%
Incremental Bank Loan | Minimum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.175%
Incremental Bank Loan | Maximum | Eurodollar
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.425%
Incremental Bank Loan | Maximum | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.425%
Japan Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 33,000,000
Remaining borrowing capacity			8,000,000
Commitment fee percentage				0.30%
Japan Credit Facility | Minimum | Tokyo Interbank Offered Rate (TIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate				1.75%
Japan Credit Facility | Maximum | Tokyo Interbank Offered Rate (TIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate				2.25%
Tsugaru Credit Facility | Tsugaru Construction Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 371,000,000
Debt term, following conversion					18 years
Tsugaru Credit Facility | Debt Service Reserve Account LC
Line of Credit Facility [Line Items]
Principal					$ 20,000,000
Tsugaru Credit Facility | O&M Reserve Account LC
Line of Credit Facility [Line Items]
Principal					$ 8,000,000
Tsugaru Credit Facility | Tsugaru Letter Of Credit Facilities
Line of Credit Facility [Line Items]
Contractual interest rate							1.10%
Unused capacity, commitment fee percentage					0.30%
Tsugaru Credit Facility | Japanese Consumption Tax Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 34,000,000
Tsugaru Credit Facility | Japanese Consumption Tax Facility | Tokyo Interbank Offered Rate (TIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate					0.30%
Bridge Loan | Gulf Wind Promissory Note
Line of Credit Facility [Line Items]
Principal			$ 22,000,000
Bridge Loan | Gulf Wind Promissory Note | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate			0.75%