Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash collateral placed with counterparties	$ 51	$ 20
Noncash collateral placed with counterparties	120	97
Cash collateral received from counterparties	45	17
Noncash collateral received from counterparties	$ 12	$ 2