Selected Balance Sheet Data (Tables)

12 Months Ended

Dec. 31, 2015

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	Dec. 31, 2015			Dec. 31, 2014
Accounts receivable, net (a)
Accounts receivable	$	61,506		$	66,217
Less allowance for bad debts	(5,128		)	(5,821		)
	$	56,378		$	60,396

Inventories

(Thousands of Dollars)	Dec. 31, 2015		Dec. 31, 2014
Inventories
Materials and supplies	$	6,785	$	6,494
Fuel	6,528		6,654
Natural gas	8,246		11,537
	$	21,559	$	24,685

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2015			Dec. 31, 2014
Property, plant and equipment, net
Electric plant	$	2,411,562		$	2,061,669
Natural gas plant	275,376			255,465
Common and other property	132,329			125,938
CWIP	65,755			231,413
Total property, plant and equipment	2,885,022			2,674,485
Less accumulated depreciation	(1,056,943		)	(1,000,204		)
	$	1,828,079		$	1,674,281