Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2015

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

^[1]

(Thousands of Dollars)	June 30, 2015			Dec. 31, 2014
Accounts receivable, net (a)
Accounts receivable	$	57,894		$	66,217
Less allowance for bad debts	(4,605		)	(5,821		)
	$	53,289		$	60,396

Inventories

(Thousands of Dollars)	June 30, 2015		Dec. 31, 2014
Inventories
Materials and supplies	$	6,914	$	6,494
Fuel	6,753		6,654
Natural gas	3,322		11,537
	$	16,989	$	24,685

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2015			Dec. 31, 2014
Property, plant and equipment, net
Electric plant	$	2,178,038		$	2,061,669
Natural gas plant	259,820			255,465
Common and other property	125,424			125,938
Construction work in progress	218,777			231,413
Total property, plant and equipment	2,782,059			2,674,485
Less accumulated depreciation	(1,029,656		)	(1,000,204		)
	$	1,752,403		$	1,674,281

[1] Accounts receivable, net includes an immaterial amount due from affiliates as of June 30, 2015 and Dec. 31, 2014, respectively.