Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2015

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2015			Dec. 31, 2014
Accounts receivable, net
Accounts receivable	$	84,326		$	77,465
Less allowance for bad debts	(5,940		)	(5,839		)
	$	78,386		$	71,626

Inventories

(Thousands of Dollars)	June 30, 2015		Dec. 31, 2014
Inventories
Materials and supplies	$	25,018	$	24,738
Fuel	7,437		18,493
	$	32,455	$	43,231

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2015			Dec. 31, 2014
Property, plant and equipment, net
Electric plant	$	5,580,225		$	5,376,606
Construction work in progress	291,565			238,519
Total property, plant and equipment	5,871,790			5,615,125
Less accumulated depreciation	(1,922,951		)	(1,871,984		)
	$	3,948,839		$	3,743,141