Selected Balance Sheet Data (Tables)

3 Months Ended

Mar. 31, 2018

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	March 31, 2018			Dec. 31, 2017
Accounts receivable, net (a)
Accounts receivable	$	74,732		$	68,073
Less allowance for bad debts	(5,354		)	(4,873		)
	$	69,378		$	63,200

Inventories

(Thousands of Dollars)	March 31, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	7,015	$	6,916
Fuel	3,842		3,866
Natural gas	798		6,976
	$	11,655	$	17,758

Property, Plant and Equipment, Net

(Thousands of Dollars)	March 31, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	2,619,336		$	2,602,671
Natural gas plant	330,055			326,723
Common and other property	182,685			181,105
Construction work in progress	178,000			148,770
Total property, plant and equipment	3,310,076			3,259,269
Less accumulated depreciation	(1,193,981		)	(1,170,541		)
	$	2,116,095		$	2,088,728