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Area of land held.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Related Party Transactions - Additional Information (Details)		1 Months Ended		3 Months Ended
	Jun. 18, 2018 USD ($)	Feb. 28, 2019 USD ($)	May 31, 2018 Megawatts	Mar. 31, 2019 USD ($) mi² Megawatts	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)
Related Party Transaction [Line Items]
Deposit balance				$ 0		$ 0
Line of credit facility fee basis points	0.105%
Credit facility outstanding amount				0
Affiliated Entity
Related Party Transaction [Line Items]
Impairments				$ 0
Vineyard Wind
Related Party Transaction [Line Items]
Leased area transmission capacity (MW) \| Megawatts				3,000
Proposed wind farm and electricity transmission project capacity (MW) \| Megawatts			800
New electric generation and transmission capacity (MW) \| Megawatts				3,200
Iberdrola Financiacion, S.A.U
Related Party Transaction [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500,000,000
Iberdrola, S.A. \| Siemens-Gamesa
Related Party Transaction [Line Items]
Business acquisition, percentage of voting interests acquired				8.10%
Related party transaction, amount				$ 2,000,000	$ 6,000,000
Networks \| New York Transco
Related Party Transaction [Line Items]
Business combination, equity interest percentage				20.00%
Portion of amount receivable from related parties				$ 1,000,000
Renewables
Related Party Transaction [Line Items]
Portion of amount receivable from related parties				$ 92,000,000
Renewables \| Vineyard Wind
Related Party Transaction [Line Items]
Business combination, equity interest percentage				50.00%
Square mileage of land containing development rights \| mi²				260
Portion of amount receivable from related parties						$ 0	$ 900,000
Amount of commitment funded to date		$ 100,000,000		$ 70,000,000