Schedule II Consolidated Valuation and Qualifying Accounts (Details) (Provision for Uncollectible Accounts [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1		$ 32,341	$ 32,884
Provision charged to income	15,735	15,551	21,839
Amounts written off, less recoveries	(36,269)	(19,208)	(22,382)
Balance at December 31			32,341
NPC [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1		29,375	30,621
Provision charged to income	13,820	13,147	17,519
Amounts written off, less recoveries	(33,497)	(16,094)	(18,765)
Balance at December 31			29,375
SPPC [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at January 1		2,966	2,262
Provision charged to income	1,915	2,404	4,321
Amounts written off, less recoveries	(2,772)	(3,114)	(3,617)
Balance at December 31			$ 2,966