Employee Benefit Plans (Paid Benefit Obligations and Accumulated Benefit Obligations in Excess of Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accumulated benefit obligation [Abstract]
Projected benefit obligations	$ 1,996	$ 1,894
Accumulated benefit obligations	1,890	1,785
Estimated fair value of plan assets	266	228
Defined Benefit Plan, Plan with Benefit Obligation in Excess of Plan Assets [Abstract]
Projected benefit obligations	2,016	10,670
Accumulated benefit obligations	1,904	10,318
Estimated fair value of plan assets	$ 285	$ 8,979