Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Collateral received from GenOn		$ 79
Cash and cash equivalents	$ 752	938	$ 748	$ 853
Funds deposited by counterparties	19	2	1	55
Restricted cash	469	446	413	414
Cash and cash equivalents, funds deposited by counterparties and restricted cash shown in the statement of cash flows	$ 1,240	$ 1,386	$ 1,162	$ 1,322