Employee Pension and Other Postretirement Benefits - Change in Plan Assets, Level 3 (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 2,934
Fair value of plan assets, end of year	3,005	$ 2,934
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	298	264
Unrealized gains (losses)	35	26
Purchases, issuances, sales, and settlements, net	5	8
Fair value of plan assets, end of year	338	298
Real estate funds | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	270	242
Unrealized gains (losses)	31	21
Purchases, issuances, sales, and settlements, net	9	7
Fair value of plan assets, end of year	310	270
Limited partnership
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	166
Fair value of plan assets, end of year	180	166
Limited partnership | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	28	22
Unrealized gains (losses)	4	5
Purchases, issuances, sales, and settlements, net	(4)	1
Fair value of plan assets, end of year	28	28
Other
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	29
Fair value of plan assets, end of year	$ 24	$ 29