Selected Balance Sheet Data (Tables)

12 Months Ended

Dec. 31, 2016

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Dec. 31, 2016			Dec. 31, 2015
Accounts receivable, net
Accounts receivable	$	80,569		$	77,054
Less allowance for bad debts	(6,379		)	(5,888		)
	$	74,190		$	71,166

Inventories

(Thousands of Dollars)	Dec. 31, 2016		Dec. 31, 2015
Inventories
Materials and supplies	$	25,453	$	24,888
Fuel	13,052		12,658
	$	38,505	$	37,546

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2016			Dec. 31, 2015
Property, plant and equipment, net
Electric plant	$	6,362,189		$	5,933,764
Construction work in progress	260,327			236,697
Total property, plant and equipment	6,622,516			6,170,461
Less accumulated depreciation	(1,926,697		)	(1,821,638		)
	$	4,695,819		$	4,348,823