Leases (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2016
Leases Disclosure [Line Items]
Property, Plant and Equipment, Net	$ 69,082	$ 54,560
Accumulated depreciation, depletion and amortization	28,384	$ 22,018
Power Purchase Arrangement [Member]
Leases Disclosure [Line Items]
Property, Plant and Equipment, Net	2,800
Accumulated depreciation, depletion and amortization	364
Rental revenue	174
Depreciation expense	$ 94
New Corporate Office [Member] | Agreement with Lessor to Construct and Lease Corporate Office Property
Leases Disclosure [Line Items]
Lease term	51 months
Required percentage payment to lessor for difference between project costs and sales proceeds	83.00%
Required percentage payment for specific full recourse events	100.00%
Required percentage payment of funded amount under certain events of default	89.90%
Lessor | 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			$ 365
Lessor | New Corporate Office [Member] | Agreement with Lessor to Construct and Lease Corporate Office Property
Leases Disclosure [Line Items]
Amount of financing commitments to fund estimated project costs	$ 465