Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash Collateral Placed with Counterparties Excluding Cash Collateral Associated with Repurchase Agreements	$ 115	$ 122
Securities Collateral Placed with Counterparties	59	72
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	5	45
Cash Collateral Received from Counterparties, Excluding Collateral Associated with Repurchase Agreements	12	10
Non-derivative Cash Collateral Received from Counterparties Associated with the Repurchase Agreements.	1
Non-cash collateral received	$ 6	$ 6