Selected Balance Sheet Data (Tables)

3 Months Ended

Mar. 31, 2014

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	March 31, 2014			Dec. 31, 2013
Accounts receivable, net
Accounts receivable	$	73,543		$	76,426
Less allowance for bad debts	(5,809		)	(5,475		)
	$	67,734		$	70,951

Inventories

(Thousands of Dollars)	March 31, 2014		Dec. 31, 2013
Inventories
Materials and supplies	$	24,011	$	21,600
Fuel	7,996		15,538
	$	32,007	$	37,138

Property, Plant and Equipment, Net

(Thousands of Dollars)	March 31, 2014			Dec. 31, 2013
Property, plant and equipment, net
Electric plant	$	4,789,975		$	4,714,398
Construction work in progress	445,751			388,323
Total property, plant and equipment	5,235,726			5,102,721
Less accumulated depreciation	(1,840,452		)	(1,818,691		)
	$	3,395,274		$	3,284,030