Employee Benefit Plans - Fair Value of Investment Categories (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.K. Plan
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 1,227	$ 1,390	$ 1,207
Not Subject to Leveling	$ 550	$ 941
Percentage of Plan Assets	100.00%	100.00%
Investment redemption notice period	60 days	60 days
U.K. Plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 57	$ 65
U.K. Plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	620	384
U.K. Plan | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	57	65
Not Subject to Leveling	$ 0	$ 0
Percentage of Plan Assets	4.60%	4.60%
U.K. Plan | Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 57	$ 65
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	664
Not Subject to Leveling	$ 363	$ 293
Percentage of Plan Assets	79.70%	47.80%
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	371
U.K. Plan | Derivatives
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	31	67
Not Subject to Leveling	$ 26	$ 54
Percentage of Plan Assets	2.60%	4.90%
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	13
U.K. Plan | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	38	42
Not Subject to Leveling	$ 38	$ 42
Percentage of Plan Assets	3.10%	3.00%
U.K. Plan | Hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 0
U.K. Plan | Hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
U.K. Plan | Diversified multi-asset funds - risk parity
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets		275
Not Subject to Leveling		$ 275
Percentage of Plan Assets		19.80%
U.K. Plan | Diversified multi-asset funds - risk parity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets		$ 0
U.K. Plan | Diversified multi-asset funds - risk parity | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets		0
U.K. Plan | Diversified multi-asset funds - dynamic asset allocation
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	123	277
Not Subject to Leveling	$ 123	$ 277
Percentage of Plan Assets	10.00%	19.90%
U.K. Plan | Diversified multi-asset funds - dynamic asset allocation | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 0
U.K. Plan | Diversified multi-asset funds - dynamic asset allocation | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	0
Qualified Plan | United States
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,582	1,764	$ 1,700
Not Subject to Leveling	$ 212	$ 295
Percentage of Plan Assets	100.00%	100.00%
Qualified Plan | United States | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 308	$ 477
Qualified Plan | United States | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	1,062	992
Qualified Plan | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	37	25
Not Subject to Leveling	$ 37	$ 25
Percentage of Plan Assets	2.30%	1.40%
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	339
Not Subject to Leveling	$ 107	$ 101
Percentage of Plan Assets	6.80%	19.20%
Qualified Plan | United States | U.S. large companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 0	$ 189
Qualified Plan | United States | U.S. large companies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	0	49
Qualified Plan | United States | U.S. small companies
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	25	37
Not Subject to Leveling	$ 0	$ 0
Percentage of Plan Assets	1.60%	2.10%
Qualified Plan | United States | U.S. small companies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 25	$ 37
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	161
Not Subject to Leveling	$ 60	$ 82
Percentage of Plan Assets	7.50%	9.10%
Qualified Plan | United States | International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 59	$ 79
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	1,198
Not Subject to Leveling	$ 8	$ 87
Percentage of Plan Assets	81.80%	68.00%
Qualified Plan | United States | Fixed income securities, global long-term debt instruments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ 223	$ 171
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	940
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	$ 1
Qualified Plan | United States | Derivatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Plan Assets	$ (1)	$ 3