Selected Balance Sheet Data (Tables)

3 Months Ended

Mar. 31, 2015

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	March 31, 2015			Dec. 31, 2014
Accounts receivable, net
Accounts receivable	$	84,817		$	77,465
Less allowance for bad debts	(6,107		)	(5,839		)
	$	78,710		$	71,626

Inventories

(Thousands of Dollars)	March 31, 2015		Dec. 31, 2014
Inventories
Materials and supplies	$	25,500	$	24,738
Fuel	6,249		18,493
	$	31,749	$	43,231

Property, Plant and Equipment, Net

(Thousands of Dollars)	March 31, 2015			Dec. 31, 2014
Property, plant and equipment, net
Electric plant	$	5,480,750		$	5,376,606
Construction work in progress	248,921			238,519
Total property, plant and equipment	5,729,671			5,615,125
Less accumulated depreciation	(1,898,627		)	(1,871,984		)
	$	3,831,044		$	3,743,141