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
Accounts receivable	$	79,982		$	80,569
Less allowance for bad debts	(6,142		)	(6,379		)
	$	73,840		$	74,190

Inventories

(Thousands of Dollars)	March 31, 2017		Dec. 31, 2016
Inventories
Materials and supplies	$	25,006	$	25,453
Fuel	6,295		13,052
	$	31,301	$	38,505

Property, Plant and Equipment, Net

(Thousands of Dollars)	March 31, 2017			Dec. 31, 2016
Property, plant and equipment, net
Electric plant	$	6,447,850		$	6,362,189
Construction work in progress	311,990			260,327
Total property, plant and equipment	6,759,840			6,622,516
Less accumulated depreciation	(1,973,290		)	(1,926,697		)
	$	4,786,550		$	4,695,819