Selected Balance Sheet Data (Tables)

3 Months Ended

Mar. 31, 2017

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	March 31, 2017			Dec. 31, 2016
Accounts receivable, net (a)
Accounts receivable	$	69,975		$	58,896
Less allowance for bad debts	(5,077		)	(4,865		)
	$	64,898		$	54,031

Inventories

(Thousands of Dollars)	March 31, 2017		Dec. 31, 2016
Inventories
Materials and supplies	$	6,798	$	6,582
Fuel	4,893		4,743
Natural gas	1,812		6,984
	$	13,503	$	18,309

Property, Plant and Equipment, Net

(Thousands of Dollars)	March 31, 2017			Dec. 31, 2016
Property, plant and equipment, net
Electric plant	$	2,520,158		$	2,499,401
Natural gas plant	299,926			294,986
Common and other property	156,213			156,316
Construction work in progress	135,927			118,822
Total property, plant and equipment	3,112,224			3,069,525
Less accumulated depreciation	(1,140,094		)	(1,121,888		)
	$	1,972,130		$	1,947,637