Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash Collateral Placed with Counterparties Excluding Cash Collateral Associated with Repurchase Agreements	$ 20	$ 122
Securities Collateral Placed with Counterparties	97	72
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	10	45
Cash Collateral Received from Counterparties, Excluding Collateral Associated with Repurchase Agreements	17	10
Non-derivative Cash Collateral received from counterparties associated with Repurchase Agreements	1
Non-cash collateral received	$ 2	$ 6