Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2017

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2017			Dec. 31, 2016
Accounts receivable, net (a)
Accounts receivable	$	58,587		$	58,896
Less allowance for bad debts	(4,247		)	(4,865		)
	$	54,340		$	54,031

Inventories

(Thousands of Dollars)	June 30, 2017		Dec. 31, 2016
Inventories
Materials and supplies	$	7,117	$	6,582
Fuel	4,615		4,743
Natural gas	3,908		6,984
	$	15,640	$	18,309

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2017			Dec. 31, 2016
Property, plant and equipment, net
Electric plant	$	2,534,258		$	2,499,401
Natural gas plant	307,217			294,986
Common and other property	170,486			156,316
Construction work in progress	145,458			118,822
Total property, plant and equipment	3,157,419			3,069,525
Less accumulated depreciation	(1,157,072		)	(1,121,888		)
	$	2,000,347		$	1,947,637