Employee Retirement Plans - Reconciliation of Change in Fair Value Measurement of Defined Benefit Plans' Consolidated Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in Plan Assets:
Fair value as of beginning of period	$ 1,435
Fair value as of end of period	1,577	$ 1,435
Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	142	144
Realized gains (losses)	16	6
Unrealized gains (losses)	1	(1)
Purchases	9	1
Sales	(37)	(8)
Settlements	0	0
Net transfers into (out of) Level 3	0	0
Fair value as of end of period	131	142
Fixed income
Change in Plan Assets:
Fair value as of beginning of period	677
Fair value as of end of period	772	677
Fixed income | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	1	1
Realized gains (losses)	0	0
Unrealized gains (losses)	0	0
Purchases	0	0
Sales	0	0
Settlements	0	0
Net transfers into (out of) Level 3	0	0
Fair value as of end of period	1	1
Other Real Estate Assets
Change in Plan Assets:
Fair value as of beginning of period	54
Fair value as of end of period	57	54
Other Real Estate Assets | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	38	37
Realized gains (losses)	1	1
Unrealized gains (losses)	2	2
Purchases	0	0
Sales	(2)	(2)
Settlements	0	0
Net transfers into (out of) Level 3	0	0
Fair value as of end of period	39	38
Limited partnerships
Change in Plan Assets:
Fair value as of beginning of period	103
Fair value as of end of period	91	103
Limited partnerships | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	103	106
Realized gains (losses)	15	5
Unrealized gains (losses)	(1)	(3)
Purchases	9	1
Sales	(35)	(6)
Settlements	0	0
Net transfers into (out of) Level 3	0	0
Fair value as of end of period	$ 91	$ 103