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Goodwill Goodwill (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Goodwill [Line Items]    
Goodwill $ 840us-gaap_Goodwill $ 840us-gaap_Goodwill
Natural Gas Distribution    
Goodwill [Line Items]    
Goodwill 746us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= cerc_NaturalGasDistributionMember		 746us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= cerc_NaturalGasDistributionMember
Energy Services    
Goodwill [Line Items]    
Goodwill 83us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= cerc_EnergyServicesMember		 83us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= cerc_EnergyServicesMember
Other    
Goodwill [Line Items]    
Goodwill $ 11us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 $ 11us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember