Property, Plant and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

Property, Pland and Equipment with Useful Life Classification

The following table summarizes the components of the electric generation and distribution assets and other property, plant and equipment (in millions) with their estimated useful lives (in years). The amounts are stated net of all prior asset impairment losses recognized.



		December 31,
	Estimated Useful Life	2017			2016
Electric generation and distribution facilities	8 - 40	$	21,529		$	22,337
Other buildings	5 - 71	1,971			1,906
Furniture, fixtures and equipment	2 - 32	284			303
Other	5 - 44	335			365
Total electric generation and distribution assets and other		24,119			24,911
Accumulated depreciation		(7,942		)	(7,919		)
Net electric generation and distribution assets and other		$	16,177		$	16,992

Interest Capitalized During Development And Construction

The following table summarizes depreciation expense (including the amortization of assets recorded under capital leases and the amortization of asset retirement obligations) and interest capitalized during development and construction on qualifying assets for the periods indicated (in millions):



Years Ended December 31,	2017		2016		2015
Depreciation expense	$	1,005	$	1,002	$	958
Interest capitalized during development and construction	139		118		84

Net Asset Value Of Regulated And Non-Regulated Assets And Accumulated Depreciation

The following table summarizes regulated and non-regulated generation and distribution property, plant and equipment and accumulated depreciation as of the dates indicated (in millions):



December 31,	2017			2016
Regulated generation, distribution assets and other, gross	$	8,093		$	7,815
Regulated accumulated depreciation	(3,357		)	(3,299		)
Regulated generation, distribution assets and other, net	4,736			4,516
Non-regulated generation, distribution assets and other, gross	16,026			17,096
Non-regulated accumulated depreciation	(4,585		)	(4,620		)
Non-regulated generation, distribution assets and other, net	11,441			12,476
Net electric generation, distribution assets and other	$	16,177		$	16,992

Schedule of Change in Asset Retirement Obligation

The following table presents amounts recognized related to asset retirement obligations for the periods indicated (in millions):



	2017			2016
Balance at January 1	$	357		$	247
Additional liabilities incurred	1			12
Liabilities settled	(21		)	(4		)
Accretion expense	16			15
Change in estimated cash flows	25			86
Other	(10		)	1
Balance at December 31	$	368		$	357