Selected Balance Sheet Data (Tables)

3 Months Ended

Mar. 31, 2016

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	March 31, 2016			Dec. 31, 2015
Accounts receivable, net (a)
Accounts receivable	$	64,138		$	61,506
Less allowance for bad debts	(4,963		)	(5,128		)
	$	59,175		$	56,378

^[1]

Inventories

(Thousands of Dollars)	March 31, 2016		Dec. 31, 2015
Inventories
Materials and supplies	$	6,676	$	6,785
Fuel	5,881		6,528
Natural gas	2,285		8,246
	$	14,842	$	21,559

Property, Plant and Equipment, Net

(Thousands of Dollars)	March 31, 2016			Dec. 31, 2015
Property, plant and equipment, net
Electric plant	$	2,431,503		$	2,411,562
Natural gas plant	276,568			275,376
Common and other property	134,327			132,329
Construction work in progress	77,148			65,755
Total property, plant and equipment	2,919,546			2,885,022
Less accumulated depreciation	(1,073,779		)	(1,056,943		)
	$	1,845,767		$	1,828,079

[1] Accounts receivable, net includes an immaterial amount due from affiliates as of March 31, 2016 and Dec. 31, 2015, respectively.