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
Accounts receivable	$	90,138		$	80,569
Less allowance for bad debts	(6,286		)	(6,379		)
	$	83,852		$	74,190

Inventories

(Thousands of Dollars)	June 30, 2017		Dec. 31, 2016
Inventories
Materials and supplies	$	25,748	$	25,453
Fuel	7,346		13,052
	$	33,094	$	38,505

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2017			Dec. 31, 2016
Property, plant and equipment, net
Electric plant	$	6,624,189		$	6,362,189
Construction work in progress	232,817			260,327
Total property, plant and equipment	6,857,006			6,622,516
Less accumulated depreciation	(1,985,215		)	(1,926,697		)
	$	4,871,791		$	4,695,819