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 (a)
Accounts receivable	$	58,896		$	61,506
Less allowance for bad debts	(4,865		)	(5,128		)
	$	54,031		$	56,378

Inventories

(Thousands of Dollars)	Dec. 31, 2016		Dec. 31, 2015
Inventories
Materials and supplies	$	6,582	$	6,785
Fuel	4,743		6,528
Natural gas	6,984		8,246
	$	18,309	$	21,559

Property, Plant and Equipment, Net

(Thousands of Dollars)	Dec. 31, 2016			Dec. 31, 2015
Property, plant and equipment, net
Electric plant	$	2,499,401		$	2,411,562
Natural gas plant	294,986			275,376
Common and other property	156,316			132,329
CWIP	118,822			65,755
Total property, plant and equipment	3,069,525			2,885,022
Less accumulated depreciation	(1,121,888		)	(1,056,943		)
	$	1,947,637		$	1,828,079