Selected Balance Sheet Data (Tables)

9 Months Ended

Sep. 30, 2015

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Sept. 30, 2015			Dec. 31, 2014
Accounts receivable, net
Accounts receivable	$	102,408		$	77,465
Less allowance for bad debts	(6,480		)	(5,839		)
	$	95,928		$	71,626

Inventories

(Thousands of Dollars)	Sept. 30, 2015		Dec. 31, 2014
Inventories
Materials and supplies	$	25,595	$	24,738
Fuel	14,285		18,493
	$	39,880	$	43,231

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2015			Dec. 31, 2014
Property, plant and equipment, net
Electric plant	$	5,673,759		$	5,376,606
Construction work in progress	338,889			238,519
Total property, plant and equipment	6,012,648			5,615,125
Less accumulated depreciation	(1,951,029		)	(1,871,984		)
	$	4,061,619		$	3,743,141