Pension and Other Postretirement Benefit Plans - Amounts included in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Amounts included in AOCI [Abstract]
Unrecognized net gain (loss)	$ (2,163)	$ (2,201)
Unrecognized prior service cost	(6)	(7)
Postretirement benefits liabilty adjustment	(2,169)	(2,208)
Expected pre-tax gains (losses) related to amortization of net actuarial gain/loss	$ (66)
Weighted average amortization period for amortization of net actuarial gain/loss	27 years
Other Postretirement Benefits [Member]
Amounts included in AOCI [Abstract]
Unrecognized net gain (loss)	$ 0	1
Unrecognized prior service cost	49	52
Postretirement benefits liabilty adjustment	49	$ 53
Expected pre-tax (gains) losses related to amortization of net prior service cost	$ (2)
Weighted average amortization period for prior service costs	27 years