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	$	53,700		$	55,039
Less allowance for bad debts	(4,116		)	(4,333		)
	$	49,584		$	50,706

Inventories

(Thousands of Dollars)	June 30, 2013		Dec. 31, 2012
Inventories
Materials and supplies	$	6,541	$	6,172
Fuel	7,268		6,664
Natural gas	3,964		6,849
	$	17,773	$	19,685

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2013			Dec. 31, 2012
Property, plant and equipment, net
Electric plant	$	1,848,303		$	1,795,239
Natural gas plant	227,790			224,625
Common and other property	112,043			111,319
Construction work in progress	77,144			62,629
Total property, plant and equipment	2,265,280			2,193,812
Less accumulated depreciation	(921,567		)	(895,576		)
	$	1,343,713		$	1,298,236