Selected Balance Sheet Data (Tables)

9 Months Ended

Sep. 30, 2017

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Sept. 30, 2017			Dec. 31, 2016
Accounts receivable, net (a)
Accounts receivable	$	55,619		$	58,896
Less allowance for bad debts	(4,405		)	(4,865		)
	$	51,214		$	54,031

Inventories

(Thousands of Dollars)	Sept. 30, 2017		Dec. 31, 2016
Inventories
Materials and supplies	$	7,417	$	6,582
Fuel	3,898		4,743
Natural gas	8,792		6,984
	$	20,107	$	18,309

Property, Plant and Equipment, Net

(Thousands of Dollars)	Sept. 30, 2017			Dec. 31, 2016
Property, plant and equipment, net
Electric plant	$	2,553,562		$	2,499,401
Natural gas plant	317,902			294,986
Common and other property	185,612			156,316
Construction work in progress	151,299			118,822
Total property, plant and equipment	3,208,375			3,069,525
Less accumulated depreciation	(1,175,847		)	(1,121,888		)
	$	2,032,528		$	1,947,637