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Leases (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
New Corporate Office | Agreement with Lessor to Construct and Lease Corporate Office Property          
Leases Disclosure [Line Items]          
Required percentage payment for specific full recourse events         100.00%
Extension term of lease         5 years
Required percentage payment to lessor for difference between project costs and sales proceeds         83.00%
Lessor | New Corporate Office | Agreement with Lessor to Construct and Lease Corporate Office Property          
Leases Disclosure [Line Items]          
Amount of financing commitments to fund estimated project costs         $ 465
Power Purchase Arrangements          
Leases Disclosure [Line Items]          
Rental revenue $ 61 $ 64 $ 146 $ 146  
Depreciation expense $ 26 $ 23 $ 76 $ 70