Selected Balance Sheet Data (Tables)

3 Months Ended

Mar. 31, 2016

Balance Sheet Related Disclosures [Abstract]

Accounts receivable, net

(Thousands of Dollars)	March 31, 2016			Dec. 31, 2015
Accounts receivable, net
Accounts receivable	$	352,249		$	313,556
Less allowance for bad debts	(19,273		)	(20,750		)
	$	332,976		$	292,806

Inventories

(Thousands of Dollars)	March 31, 2016		Dec. 31, 2015
Inventories
Materials and supplies	$	208,090	$	200,888
Fuel	88,287		104,499
Natural gas	12,201		38,529
	$	308,578	$	343,916

Property, plant and equipment, net

(Thousands of Dollars)	March 31, 2016			Dec. 31, 2015
Property, plant and equipment, net
Electric plant	$	16,298,761		$	16,256,887
Natural gas plant	1,249,933			1,248,408
Common and other property	622,749			624,409
Construction work in progress	650,693			545,535
Total property, plant and equipment	18,822,136			18,675,239
Less accumulated depreciation	(6,354,488		)	(6,251,498		)
Nuclear fuel	2,450,363			2,447,251
Less accumulated amortization	(2,089,404		)	(2,063,654		)
	$	12,828,607		$	12,807,338