Employee Retirement Plans (Postretirement Benefits) (Details) - Postretirement Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation
Benefit obligation at beginning of year	$ 776	$ 763
Service cost	10	9	$ 13
Interest cost	26	26	33
Plan participants’ contributions	8	8
Actuarial loss (gain)	17	16
Other		2
Benefits paid	(50)	(50)
Medicare subsidy received	2	2
Benefit obligation at end of year	789	776	763
Funded status at end of year
Benefit obligations	$ (776)	$ (763)	$ (763)	$ (789)	$ (776)
Funded status of plans				(789)	(776)
Amounts recognized in the consolidated balance sheets consist of:
Current liability				(40)	(39)
Noncurrent liability				(749)	(737)
Recognized liability				(789)	(776)
Amounts recognized in accumulated other comprehensive income consist of:
Net actuarial loss				68	50
Prior service credit				(12)	(15)
Amount recognized at end of year				$ 56	$ 35