Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	$ 2,023	$ 2,236
Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	237	350
Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,786	1,886
Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	188	176
Cash and short-term investments [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	8	17
Cash and short-term investments [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	180	159
Cash and short-term investments [Member] | Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	312	293
US indexed equity securities [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	63	88
US indexed equity securities [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	249	205
US indexed equity securities [Member] | Level 3 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
US growth and value securities [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	132	204
US growth and value securities [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	92	147
US growth and value securities [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	40	57
US growth and value securities [Member] | Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	47	67
UK equity securities [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	34	56
UK equity securities [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	13	11
UK equity securities [Member] | Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	124	139
International equity securities excluding UK [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	40	42
International equity securities excluding UK [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	84	97
International equity securities excluding UK [Member] | Level 3 [Member]
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 bonds [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,072	1,165
Long duration bonds [Member] | Level 1 [Member]
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 bonds [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	1,072	1,165
Long duration bonds [Member] | Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	33	25
Intermediate Duration Securities [Member] | Level 1 [Member]
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 [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	33	25
Intermediate Duration Securities [Member] | Level 3 [Member]
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	6	6
Agency mortgage backed securities | Level 1 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	6	6
Agency mortgage backed securities | Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	17	18
Asset-backed Securities [Member] | Level 1 [Member]
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 [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	17	18
Asset-backed Securities [Member] | Level 3 [Member]
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 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	23	37
Non-agency mortgage backed securities [Member] | Level 1 [Member]
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 [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	23	37
Non-agency mortgage backed securities [Member] | Level 3 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
UK fixed income securities [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	6	7
UK fixed income securities [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
UK fixed income securities [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	6	7
UK fixed income securities [Member] | Level 3 [Member]
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 excluding UK [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	48	85
International fixed income securities excluding UK [Member] | Level 1 [Member]
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 excluding UK [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	48	85
International fixed income securities excluding UK [Member] | Level 3 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Other [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	15	14
Other [Member] | Level 1 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	0	0
Other [Member] | Level 2 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	15	14
Other [Member] | Level 3 [Member]
Fair value of defined benefit plans assets by asset class
Fair value of defined benefit plans assets by asset class, total	$ 0	$ 0