Fair Value of Financial Instruments (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Customers' advances for construction	$ 66,143	$ 66,966
Carrying Amount [Member]
Long-term debt	1,559,027	1,560,389
Estimated Fair Value [Member]
Long-term debt	$ 1,586,315	$ 1,483,816

For utilities only, represents the carrying amount of the liability as of the balance sheet date for payments received by a utility from its customers in advance of performing its obligations under terms of its construction agreements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 910
-SubTopic 405
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6471548&loc=d3e50628-109371

Name:	us-gaap_CustomerAdvancesForConstruction
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant