Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2018

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$	107,990		$	85,929
Less allowance for bad debts	(5,611		)	(6,348		)
	$	102,379		$	79,581

Inventories

(Thousands of Dollars)	June 30, 2018		Dec. 31, 2017
Inventories
Materials and supplies	$	25,416	$	26,218
Fuel	7,102		14,215
	$	32,518	$	40,433

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2018			Dec. 31, 2017
Property, plant and equipment, net
Electric plant	$	7,059,344		$	6,765,371
Construction work in progress	447,945			351,875
Total property, plant and equipment	7,507,289			7,117,246
Less accumulated depreciation	(2,073,102		)	(2,021,637		)
	$	5,434,187		$	5,095,609