Description of Business Description of Business - Cash and Available Revolver Capacity (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 1,515	$ 1,513
Reportable Legal Entities [Member] | CERP
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	264
Revolver capacity	270
Revolver capacity drawn or committed to letters of credit	0
Total	534
Reportable Legal Entities [Member] | CGP
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1,035
Revolver capacity	160
Revolver capacity drawn or committed to letters of credit	0
Total	1,195
Other
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	125
Revolver capacity	0
Revolver capacity drawn or committed to letters of credit	0
Total	125
Other | CES
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	91
Revolver capacity	0
Revolver capacity drawn or committed to letters of credit	0
Total	$ 91