Selected Balance Sheet Data (Tables)

12 Months Ended

Dec. 31, 2017

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Dec. 31, 2017			Dec. 31, 2016
Accounts receivable, net
Accounts receivable	$	85,929		$	80,569
Less allowance for bad debts	(6,348		)	(6,379		)
	$	79,581		$	74,190

Inventories

(Thousands of Dollars)	Dec. 31, 2017		Dec. 31, 2016
Inventories
Materials and supplies	$	26,218	$	25,453
Fuel	14,215		13,052
	$	40,433	$	38,505

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2017			Dec. 31, 2016
Property, plant and equipment, net
Electric plant	$	6,765,371		$	6,362,189
Construction work in progress	351,875			260,327
Total property, plant and equipment	7,117,246			6,622,516
Less accumulated depreciation	(2,021,637		)	(1,926,697		)
	$	5,095,609		$	4,695,819