Share-based Compensation and Employee Benefits - Benefit Obligation, Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of period	$ 2,566	$ 2,982
Service cost	53	52	$ 40
Interest cost	96	84	88
Actuarial loss (gain)	159	(172)
Benefits paid	(178)	(175)
Participant contributions	0	0
Medicare Part D subsidy receipts	0	0
Other	0	(205)
Benefit obligation at end of period	2,696	2,566	2,982
Change in plan assets:
Fair value of plan assets at beginning of period	1,864	2,296
Actual return on plan assets	330	(128)
Employer contributions	60	30
Participant contributions	0	0
Medicare Part D subsidy receipts	0	0
Benefits paid	(178)	(175)
Other	0	(159)
Fair value of plan assets at end of period	2,076	1,864	2,296
Funded status - net liability at December 31,	(620)	(702)
OPEB
Change in benefit obligation:
Benefit obligation at beginning of period	339	425
Service cost	1	1	1
Interest cost	12	12	13
Actuarial loss (gain)	10	(53)
Benefits paid	(32)	(33)
Participant contributions	2	1
Medicare Part D subsidy receipts	1	1
Other	0	(15)
Benefit obligation at end of period	333	339	425
Change in plan assets:
Fair value of plan assets at beginning of period	306	335
Actual return on plan assets	49	(5)
Employer contributions	7	7
Participant contributions	2	1
Medicare Part D subsidy receipts	1	1
Benefits paid	(32)	(33)
Other	0	0
Fair value of plan assets at end of period	333	306	$ 335
Funded status - net liability at December 31,	$ 0	$ (33)