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, 2015	Dec. 31, 2014
Change in Plan Assets:
Fair value as of beginning of period	$ 1,340
Fair value as of end of period	1,301	$ 1,340
Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	139	75
Realized gains	(1)	1
Unrealized gains (losses)	(6)	1
Purchases	2	67
Sales	(2)	(3)
Settlements	0	0
Net transfers into (out of) Level 3	0	0
Fair value as of end of period	144	139
Fixed income
Change in Plan Assets:
Fair value as of beginning of period	571
Fair value as of end of period	577	571
Fixed income | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	1	1
Realized gains	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	55
Fair value as of end of period	52	55
Other Real Estate Assets | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	35	35
Realized gains	(1)	1
Unrealized gains (losses)	(5)	(1)
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	37	35
Limited partnerships
Change in Plan Assets:
Fair value as of beginning of period	103
Fair value as of end of period	106	103
Limited partnerships | Significant Unobservable Inputs (Level 3)
Change in Plan Assets:
Fair value as of beginning of period	103	39
Realized gains	0	0
Unrealized gains (losses)	(1)	2
Purchases	2	67
Sales	0	(1)
Settlements	0	0
Net transfers into (out of) Level 3	0	0
Fair value as of end of period	$ 106	$ 103