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
Accounts receivable	$	77,465		$	76,426
Less allowance for bad debts	(5,839		)	(5,475		)
	$	71,626		$	70,951

Inventories

(Thousands of Dollars)	Dec. 31, 2014		Dec. 31, 2013
Inventories
Materials and supplies	$	24,738	$	21,600
Fuel	18,493		15,538
	$	43,231	$	37,138

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2014			Dec. 31, 2013
Property, plant and equipment, net
Electric plant	$	5,376,606		$	4,714,398
Construction work in progress	238,519			388,323
Total property, plant and equipment	5,615,125			5,102,721
Less accumulated depreciation	(1,871,984		)	(1,818,691		)
	$	3,743,141		$	3,284,030