Employee Benefit Plans - Fair Value of Investment Categories (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017
United States
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 1,582	$ 1,863	$ 1,764
U.K. Plan
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,227	1,362	$ 1,390
Not Subject to Leveling	$ 550	$ 563
Percentage of Plan Assets	100.00%	100.00%
Investment redemption notice period	60 days
U.K. Plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 57	$ 34
U.K. Plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	620	765
U.K. Plan | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	57	34
Not Subject to Leveling	$ 0	$ 0
Percentage of Plan Assets	4.60%	2.50%
U.K. Plan | Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 57	$ 34
U.K. Plan | Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
U.K. Plan | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	978	1,247
Not Subject to Leveling	$ 363	$ 474
Percentage of Plan Assets	79.70%	91.60%
U.K. Plan | Fixed income securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 0
U.K. Plan | Fixed income securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	615	773
U.K. Plan | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	31	81
Not Subject to Leveling	$ 26	$ 89
Percentage of Plan Assets	2.60%	5.90%
U.K. Plan | Derivatives | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 0
U.K. Plan | Derivatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	5	(8)
U.K. Plan | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	38
Not Subject to Leveling	$ 38
Percentage of Plan Assets	3.10%
U.K. Plan | Hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0
U.K. Plan | Hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0
U.K. Plan | Dynamic asset allocation
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	123
Not Subject to Leveling	$ 123
Percentage of Plan Assets	10.00%
U.K. Plan | Dynamic asset allocation | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0
U.K. Plan | Dynamic asset allocation | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0
Qualified Plan | United States
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,582	1,863
Not Subject to Leveling	$ 212	$ 244
Percentage of Plan Assets	100.00%	100.00%
Qualified Plan | United States | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 308	$ 507
Qualified Plan | United States | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,062	1,112
Qualified Plan | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	37	24
Not Subject to Leveling	$ 37	$ 24
Percentage of Plan Assets	2.30%	1.30%
Qualified Plan | United States | Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 0
Qualified Plan | United States | Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
Qualified Plan | United States | U.S. large companies
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	107	140
Not Subject to Leveling	$ 107	$ 140
Percentage of Plan Assets	6.80%	7.50%
Qualified Plan | United States | U.S. large companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 0
Qualified Plan | United States | U.S. large companies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
Qualified Plan | United States | U.S. small companies
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	25	30
Not Subject to Leveling	$ 0	$ 0
Percentage of Plan Assets	1.60%	1.60%
Qualified Plan | United States | U.S. small companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 25	$ 30
Qualified Plan | United States | U.S. small companies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
Qualified Plan | United States | International
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	119	147
Not Subject to Leveling	$ 60	$ 75
Percentage of Plan Assets	7.50%	7.90%
Qualified Plan | United States | International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 59	$ 72
Qualified Plan | United States | International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
Qualified Plan | United States | Fixed income securities, global long-term debt instruments
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,294	1,518
Not Subject to Leveling	$ 8	$ 5
Percentage of Plan Assets	81.80%	81.50%
Qualified Plan | United States | Fixed income securities, global long-term debt instruments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 223	$ 405
Qualified Plan | United States | Fixed income securities, global long-term debt instruments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,063	1,108
Qualified Plan | United States | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	4
Not Subject to Leveling	$ 0	$ 0
Percentage of Plan Assets	0.00%	0.20%
Qualified Plan | United States | Derivatives | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 1	$ 0
Qualified Plan | United States | Derivatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ (1)	$ 4