Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2013

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2013			Dec. 31, 2012
Accounts receivable, net
Accounts receivable	$	294,264		$	299,379
Less allowance for bad debts	(21,276		)	(21,918		)
	$	272,988		$	277,461

Inventories

(Thousands of Dollars)	June 30, 2013		Dec. 31, 2012
Inventories
Materials and supplies	$	56,452	$	54,486
Fuel	78,263		89,246
Natural gas	56,956		80,062
	$	191,671	$	223,794

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2013			Dec. 31, 2012
Property, plant and equipment, net
Electric plant	$	9,990,142		$	9,782,163
Natural gas plant	2,638,057			2,583,394
Common and other property	753,157			761,712
Plant to be retired (a)	115,466			152,730
Construction work in progress	720,008			506,225
Total property, plant and equipment	14,216,830			13,786,224
Less accumulated depreciation	(3,895,194		)	(3,755,233		)
	$	10,321,636		$	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.