Selected Balance Sheet Data (Tables)

12 Months Ended

Dec. 31, 2014

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Dec. 31, 2014		Dec. 31, 2013
Accounts receivable, net (a)
Accounts receivable	66,217		64,180
Less allowance for bad debts	(5,821	)	(4,911	)
	60,396		59,269

Inventories

(Thousands of Dollars)	Dec. 31, 2014		Dec. 31, 2013
Inventories
Materials and supplies	$	6,494	$	6,437
Fuel	6,654		5,915
Natural gas	11,537		9,123
	$	24,685	$	21,475

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2014			Dec. 31, 2013
Property, plant and equipment, net
Electric plant	$	2,061,669		$	1,913,354
Natural gas plant	255,465			236,047
Common and other property	125,938			112,886
CWIP	231,413			127,954
Total property, plant and equipment	2,674,485			2,390,241
Less accumulated depreciation	(1,000,204		)	(947,462		)
	$	1,674,281		$	1,442,779