Pension and Other Postretirement Benefits - Funded Status (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2017	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits [Member]
Change in benefit obligation [Abstract]
Benefit obligation, January 1		$ 4,934		$ 5,269
Service cost		2		2		$ 2
Interest cost		199		194		206
(Gain) loss from past experience		57	[1]	(239)	[2]
Benefits paid from plan assets		(284)		(270)
Benefits paid - other		(20)		(22)
Benefit obligation, December 31		4,888		4,934		5,269
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1		3,981		4,170
Actual return on plan assets		279		75
Benefits paid		(284)		(270)
Contributions	$ 150	1		6
Fair value of plan assets, December 31		3,977		3,981		4,170
Funded Status		(911)		(953)
Expected total pension plan contributions for 2017		150
Other Postretirement Benefits [Member]
Change in benefit obligation [Abstract]
Benefit obligation, January 1		295		335
Service cost		0		0		0
Interest cost		11		11		12
(Gain) loss from past experience		2		(19)
Benefits paid from plan assets		(3)		(3)
Benefits paid - other		(28)		(29)
Benefit obligation, December 31		277		295		335
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1		8		12
Actual return on plan assets		0		(1)
Benefits paid		(3)		(3)
Contributions		0		0
Fair value of plan assets, December 31		5		8		$ 12
Funded Status		$ (272)		$ (287)

[1] Positive amount reflects a decrease in the discount rate, partially offset by a favorable change in the mortality assumption.

[2] Negative amount reflects an increase in the discount rate and a favorable change in the mortality assumption.