Segment Reporting Segment Reporting - Condensed Balance Sheets - By Segment (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 14,793	$ 14,894
Total liabilities	18,150	16,312
Operating Segments | CERP
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	6,980	6,941
Total liabilities	5,867	5,903
Operating Segments | CGP
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	7,181	7,353
Total liabilities	2,644	2,709
Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,197	1,246
Total liabilities	9,712	7,758
Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	(565)	(646)
Total liabilities	$ (73)	$ (58)