Employee Benefits - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Total recognized	$ (25)	$ 215	$ (231)
Retirement Plans [Member]
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	6	5	10
Interest cost	96	99	91
Expected return on assets	(127)	(138)	(129)
Actuarial loss (gain)	20	11	26
Prior service cost (credit)	0	0	5
Curtailment	0	0	(8)
Net periodic benefit cost	(5)	(23)	(5)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial (gain) loss	(6)	232	(213)
Recognized actuarial (gain) loss	(13)	(7)	(15)
Prior service cost (credit)	0	0	5
Total recognized	(19)	225	(223)
Postretirement Plans [Member]
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	0	1	2
Interest cost	$ 3	4	5
Expected return on assets		0	0
Actuarial loss (gain)	$ 0	(1)	0
Prior service cost (credit)	(1)	0	(1)
Curtailment	0	(1)	(12)
Net periodic benefit cost	2	3	(6)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial (gain) loss	(17)	3	(8)
Recognized actuarial (gain) loss	0	1	0
Prior service cost (credit)	1	(5)	0
Total recognized	$ (16)	$ (1)	$ (8)