Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash Collateral Placed with Counterparties Excluding Cash Collateral Associated with Repurchase Agreements	$ 28	$ 20
Securities Collateral Placed with Counterparties	100	97
Cash Collateral Received from Counterparties, Excluding Collateral Associated with Repurchase Agreements	30	17
Non-cash collateral received	$ 2	$ 2