Selected Balance Sheet Data (Tables)

12 Months Ended

Dec. 31, 2013

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Dec. 31, 2013			Dec. 31, 2012
Accounts receivable, net
Accounts receivable	$	76,426		$	66,789
Less allowance for bad debts	(5,475		)	(4,722		)
	$	70,951		$	62,067

Inventories

(Thousands of Dollars)	Dec. 31, 2013		Dec. 31, 2012
Inventories
Materials and supplies	$	21,600	$	18,129
Fuel	15,538		13,208
	$	37,138	$	31,337

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2013			Dec. 31, 2012
Property, plant and equipment, net
Electric plant	$	4,714,398		$	4,379,208
Construction work in progress	388,323			237,136
Total property, plant and equipment	5,102,721			4,616,344
Less accumulated depreciation	(1,818,691		)	(1,754,588		)
	$	3,284,030		$	2,861,756