Selected Balance Sheet Data (Tables)

9 Months Ended

Sep. 30, 2014

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Sept. 30, 2014			Dec. 31, 2013
Accounts receivable, net
Accounts receivable	$	56,852		$	62,585
Less allowance for bad debts	(5,344		)	(4,911		)
	$	51,508		$	57,674

Inventories

(Thousands of Dollars)	Sept. 30, 2014		Dec. 31, 2013
Inventories
Materials and supplies	$	6,892	$	6,437
Fuel	6,706		5,915
Natural gas	13,049		9,123
	$	26,647	$	21,475

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2014			Dec. 31, 2013
Property, plant and equipment, net
Electric plant	$	2,003,119		$	1,913,354
Natural gas plant	244,244			236,047
Common and other property	115,199			112,886
Construction work in progress	210,445			127,954
Total property, plant and equipment	2,573,007			2,390,241
Less accumulated depreciation	(985,564		)	(947,462		)
	$	1,587,443		$	1,442,779