Segment Reporting Segment Reporting - Condensed Balance Sheets - By Segment (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 14,353	$ 14,894
Total liabilities	18,168	16,312
Operating Segments | CERP
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	7,025	6,941
Total liabilities	5,855	5,903
Operating Segments | CGP
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	6,694	7,353
Total liabilities	2,250	2,709
Other
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,104	1,246
Total liabilities	10,154	7,758
Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	(470)	(646)
Total liabilities	$ (91)	$ (58)