Selected Balance Sheet Data (Tables)

9 Months Ended

Sep. 30, 2018

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Sept. 30, 2018			Dec. 31, 2017
Accounts receivable, net (a)
Accounts receivable	$	56,498		$	68,073
Less allowance for bad debts	(5,074		)	(4,873		)
	$	51,424		$	63,200

Inventories

(Thousands of Dollars)	Sept. 30, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	6,770	$	6,916
Fuel	3,718		3,866
Natural gas	7,897		6,976
	$	18,385	$	17,758

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	2,653,711		$	2,602,671
Natural gas plant	339,399			326,723
Common and other property	188,934			181,105
Construction work in progress	235,065			148,770
Total property, plant and equipment	3,417,109			3,259,269
Less accumulated depreciation	(1,226,849		)	(1,170,541		)
	$	2,190,260		$	2,088,728