Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2018

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Accounts receivable, net (a)
Accounts receivable	$	65,606		$	68,073
Less allowance for bad debts	(4,726		)	(4,873		)
	$	60,880		$	63,200

Inventories

(Thousands of Dollars)	June 30, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	6,879	$	6,916
Fuel	3,780		3,866
Natural gas	3,139		6,976
	$	13,798	$	17,758

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	2,632,764		$	2,602,671
Natural gas plant	334,329			326,723
Common and other property	187,120			181,105
Construction work in progress	205,502			148,770
Total property, plant and equipment	3,359,715			3,259,269
Less accumulated depreciation	(1,209,419		)	(1,170,541		)
	$	2,150,296		$	2,088,728