Employee Pension and Other Postretirement Benefits - Changes in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan and Other Postretirement Benefits Plan
Recognized net actuarial gain (loss)	$ 121	$ 62	$ 116
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Net Actuarial (Gain) Loss, Balance	1,044	467	1,042
Recognized net actuarial gain (loss)	89	636	(468)
Net Actuarial (Gain) Loss, Recognized in net income during the year	(106)	(59)	(107)
Net Actuarial (Gain) Loss, Balance	1,027	1,044	467
Prior Service Credit, beginning balance	(138)
Prior Service Credit, Arising during the year	0		(150)
Prior Service Credit, recognized in net income during the year	17		12
Prior Service Credit, ending balance	(121)	(138)
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Net Actuarial (Gain) Loss, Balance	62	17	80
Recognized net actuarial gain (loss)	32	46	(56)
Net Actuarial (Gain) Loss, Recognized in net income during the year	(12)	(1)	(7)
Net Actuarial (Gain) Loss, Balance	82	62	17
Prior Service Credit, beginning balance	(23)
Prior Service Credit, Arising during the year	0		(28)
Prior Service Credit, recognized in net income during the year	8		$ 5
Prior Service Credit, ending balance	$ (15)	$ (23)