Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	$ 1,960	$ 1,987
Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	1,262	1,291
Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	82	94
Level 2
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	1,141	1,158
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	698	696
Cash and short-term investments | Total
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	3	4
Cash and short-term investments | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	3	4
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	23	21
U.S. indexes | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	23	21
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	56	69
U.S. growth and value | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets	56	69
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		0
U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		0
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		0
International, excluding U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of plan assets		0
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	1,078	1,070
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	1,078	1,070
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	20	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	20	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	3	4
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	3	4
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	14	18
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	14	18
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	11	13
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	11	13
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	15	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	15	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