Fair Value of Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount			$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount			172
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount			0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount			$ 172
Maximum [Member]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 1	$ 1
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	1	1
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	1	1
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	$ 1	$ 1