Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair value of defined benefit plans assets by asset class
Collective investment funds	$ 749	$ 763
Fair value of plan assets, excluding collective investment funds	1,324	1,260
Fair value of defined benefit plans assets by asset class, total	2,073	2,023
Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	251	197
Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,062	1,063
Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	11	0
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	38	1
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	38	1
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	69	63
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	69	63
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	103	92
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	103	92
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	3	6
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	3	6
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	38	35
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	38	35
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	970	969
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	970	969
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	32	31
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	32	31
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	6
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	6
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	17
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	17
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	20	23
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	20	23
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	16	17
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	16	17
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
Real estate funds
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	11
Real estate funds | Level 1
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0
Real estate funds | Level 2
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0
Real estate funds | Level 3
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	11
Other
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Other | 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
Other | 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
Other | 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