Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair value of defined benefit plans assets by asset class
Collective investment funds	$ 793	$ 749
Fair value of plan assets, excluding collective investment funds	1,426	1,324
Fair value of defined benefit plans assets by asset class, total	2,219	2,073
Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	219	251
Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,168	1,062
Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	39	11
Cash and short-term investments
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	10	38
Cash and short-term investments | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	10	38
Cash and short-term investments | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Cash and short-term investments | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
US indexed equity securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	50	69
US indexed equity securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	50	69
US indexed equity securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
US indexed equity securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
U.S. growth and value
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	109	103
U.S. growth and value | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	109	103
U.S. growth and value | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
U.S. growth and value | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
UK equity securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	5	3
UK equity securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	5	3
UK equity securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
UK equity securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
International equity securities excluding UK
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	45	38
International equity securities excluding UK | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	45	38
International equity securities excluding UK | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
International equity securities excluding UK | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Long duration strategy
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,076	970
Long duration strategy | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Long duration strategy | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,076	970
Long duration strategy | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Intermediate duration securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	35	32
Intermediate duration securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Intermediate duration securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	35	32
Intermediate duration securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Agency mortgage backed securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	5	5
Agency mortgage backed securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Agency mortgage backed securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	5	5
Agency mortgage backed securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Asset-backed securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	19	19
Asset-backed securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Asset-backed securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	19	19
Asset-backed securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Non-agency mortgage backed securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	15	20
Non-agency mortgage backed securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Non-agency mortgage backed securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	15	20
Non-agency mortgage backed securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
International fixed income securities
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	18	16
International fixed income securities | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
International fixed income securities | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	18	16
International fixed income securities | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
U.K.
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	39	11
U.K. | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
U.K. | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
U.K. | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	$ 39	$ 11