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	$	96,110		$	76,426
Less allowance for bad debts	(5,794		)	(5,475		)
	$	90,316		$	70,951

Inventories

(Thousands of Dollars)	Sept. 30, 2014		Dec. 31, 2013
Inventories
Materials and supplies	$	23,770	$	21,600
Fuel	19,625		15,538
	$	43,395	$	37,138

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2014			Dec. 31, 2013
Property, plant and equipment, net
Electric plant	$	5,200,187		$	4,714,398
Construction work in progress	264,894			388,323
Total property, plant and equipment	5,465,081			5,102,721
Less accumulated depreciation	(1,848,891		)	(1,818,691		)
	$	3,616,190		$	3,284,030