Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash collateral placed with counterparties	$ 19	$ 26
Noncash collateral placed with counterparties	99	105
Cash collateral received from counterparties	$ 12	30
Noncash collateral received from counterparties		$ 3