Selected Balance Sheet Data (Tables)

9 Months Ended

Sep. 30, 2013

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Sept. 30, 2013			Dec. 31, 2012
Accounts receivable, net
Accounts receivable	$	305,360		$	299,379
Less allowance for bad debts	(22,537		)	(21,918		)
	$	282,823		$	277,461

Inventories

(Thousands of Dollars)	Sept. 30, 2013		Dec. 31, 2012
Inventories
Materials and supplies	$	55,869	$	54,486
Fuel	86,597		89,246
Natural gas	108,768		80,062
	$	251,234	$	223,794

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2013			Dec. 31, 2012
Property, plant and equipment, net
Electric plant	$	10,045,683		$	9,782,163
Natural gas plant	2,662,749			2,583,394
Common and other property	749,437			761,712
Plant to be retired (a)	115,753			152,730
Construction work in progress	850,106			506,225
Total property, plant and equipment	14,423,728			13,786,224
Less accumulated depreciation	(3,921,853		)	(3,755,233		)
	$	10,501,875		$	10,030,991

^(a)  In 2010, in response to the Clean Air Clean Jobs Act (CACJA), the Colorado Public Utilities Commission (CPUC) approved the early retirement of Cherokee Units 1, 2 and 3, Arapahoe Unit 3 and Valmont Unit 5 between 2011 and 2017.  In 2011, Cherokee Unit 2 was retired and in 2012, Cherokee Unit 1 was retired.  Amounts are presented net of accumulated depreciation.