PROPERTY, PLANT AND EQUIPMENT

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	2015			2014
Electric generation and distribution facilities	5 - 68	$	25,427		$	27,488
Other buildings	3 - 53	1,868			1,694
Furniture, fixtures and equipment	2 - 31	305			307
Other	1 - 50	891			970
Total electric generation and distribution assets and other		28,491			30,459
Accumulated depreciation		(9,449		)	(9,962		)
Net electric generation and distribution assets and other ⁽¹⁾		$	19,042		$	20,497


⁽¹⁾	Net electric generation and distribution assets and other include unamortized internal-use software costs of $83 million and $115 million as of December 31, 2015 and 2014, respectively.

The following table summarizes depreciation expense (including the amortization of assets recorded under capital leases), amortization of internal-use software and interest capitalized during development and construction on qualifying assets for the periods indicated (in millions):



Year Ended December 31,	2015		2014		2013
Depreciation expense (including amortization of assets recorded under capital leases)	$	1,104	$	1,204	$	1,193
Amortization of internal-use software	29		33		36
Interest capitalized during development and construction	90		120		84

Property, plant and equipment, net of accumulated depreciation, of $12 billion and $15 billion was mortgaged, pledged or subject to liens as of December 31, 2015 and 2014, respectively.

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



December 31,	2015			2014
Regulated generation, distribution assets and other, gross	$	11,818		$	13,103
Regulated accumulated depreciation	(4,351		)	(4,841		)
Regulated generation, distribution assets and other, net	7,467			8,262
Non-regulated generation, distribution assets and other, gross	16,673			17,356
Non-regulated accumulated depreciation	(5,098		)	(5,121		)
Non-regulated generation, distribution assets and other, net	11,575			12,235
Net electric generation, distribution assets and other	$	19,042		$	20,497

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



	2015			2014
Balance at January 1	$	209		$	142
Additional liabilities incurred	43			51
Liabilities settled	(6		)	(11		)
Accretion expense	13			12
Change in estimated cash flows	(7		)	15
Other	(5		)	—
Balance at December 31	$	247		$	209

The Company's asset retirement obligations covered by the relevant guidance primarily include active ash landfills, water treatment basins and the removal or dismantlement of certain plants and equipment. There were $2 million of legally restricted assets for the year ended December 31, 2015 and none for the year ended December 31, 2014 for purposes of settling asset retirement obligations.

Ownership of Certain Coal-Fired Facilities

DP&L has undivided ownership interests in five coal-fired generation facilities jointly owned with other utilities. DP&L's share of the operating costs of such facilities is included in Cost of Sales in the Consolidated Statements of Operations and its share of investment in the facilities is included in Property, Plant and Equipment in the Consolidated Balance Sheets. DP&L's undivided ownership interest in such facilities at December 31, 2015 is as follows:



	DP&L Share		DP&L Investment
	Ownership		Gross Plant In Service		Accumulated Depreciation		Construction Work In Process
Production units:			($ in millions)
Conesville Unit 4	17	%	$	26	$	4	$	1
Killen Station	67	%	342		29		2
Miami Fort Units 7 and 8	36	%	219		32		6
Stuart Station	35	%	236		19		18
Zimmer Station	28	%	188		44		12
Transmission	various		43		8		—
Total			$	1,054	$	136	$	39