Selected Balance Sheet Data (Tables)

6 Months Ended

Jun. 30, 2014

Balance Sheet Related Disclosures [Abstract]

Accounts Receivable, Net

(Thousands of Dollars)	June 30, 2014			Dec. 31, 2013
Accounts receivable, net
Accounts receivable	$	56,469		$	62,585
Less allowance for bad debts	(4,785		)	(4,911		)
	$	51,684		$	57,674

Inventories

(Thousands of Dollars)	June 30, 2014		Dec. 31, 2013
Inventories
Materials and supplies	$	6,882	$	6,437
Fuel	7,071		5,915
Natural gas	4,850		9,123
	$	18,803	$	21,475

Property, Plant and Equipment, Net

(Thousands of Dollars)	June 30, 2014			Dec. 31, 2013
Property, plant and equipment, net
Electric plant	$	1,984,497		$	1,913,354
Natural gas plant	239,659			236,047
Common and other property	114,063			112,886
Construction work in progress	154,992			127,954
Total property, plant and equipment	2,493,211			2,390,241
Less accumulated depreciation	(971,382		)	(947,462		)
	$	1,521,829		$	1,442,779