Selected Balance Sheet Data (Tables)

9 Months Ended

Sep. 30, 2016

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Sept. 30, 2016			Dec. 31, 2015
Accounts receivable, net
Accounts receivable	$	97,640		$	77,054
Less allowance for bad debts	(6,341		)	(5,888		)
	$	91,299		$	71,166

Inventories

(Thousands of Dollars)	Sept. 30, 2016		Dec. 31, 2015
Inventories
Materials and supplies	$	25,440	$	24,888
Fuel	13,597		12,658
	$	39,037	$	37,546

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2016			Dec. 31, 2015
Property, plant and equipment, net
Electric plant	$	6,191,562		$	5,933,764
Construction work in progress	294,025			236,697
Total property, plant and equipment	6,485,587			6,170,461
Less accumulated depreciation	(1,906,368		)	(1,821,638		)
	$	4,579,219		$	4,348,823