Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	$ 1,987	$ 2,219
Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	1,291	1,426
Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	94	219
Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	1,158	1,168
Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	39	39
Fair Value Measured at Net Asset Value Per Share
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	696	793
Cash and short-term investments | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	4	10
Cash and short-term investments | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	4	10
Cash and short-term investments | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Cash and short-term investments | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.S. indexes | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	21	50
U.S. indexes | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	21	50
U.S. indexes | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.S. indexes | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.S. growth and value | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	69	109
U.S. growth and value | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	69	109
U.S. growth and value | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.S. growth and value | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.K. | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		5
U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		5
U.K. | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		0
U.K. | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		0
International, excluding U.K. | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		45
International, excluding U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		45
International, excluding U.K. | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		0
International, excluding U.K. | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		0
Long duration strategy | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	1,070	1,076
Long duration strategy | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Long duration strategy | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	1,070	1,076
Long duration strategy | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Intermediate duration securities | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	35	35
Intermediate duration securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Intermediate duration securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	35	35
Intermediate duration securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Agency mortgage backed securities | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	4	5
Agency mortgage backed securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Agency mortgage backed securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	4	5
Agency mortgage backed securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Asset backed securities | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	18	19
Asset backed securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Asset backed securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	18	19
Asset backed securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Non-agency mortgage backed securities | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	13	15
Non-agency mortgage backed securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
Non-agency mortgage backed securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	13	15
Non-agency mortgage backed securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
International, excluding U.K. | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	18	18
International, excluding U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
International, excluding U.K. | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	18	18
International, excluding U.K. | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.K. | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	39	39
U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.K. | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	0	0
U.K. | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	$ 39	$ 39