Fair value measurements Assets Held in Trusts for Retirement Benefits (Details) - Fair value measurements on a recurring basis - USD ($) $ in Millions	Dec. 31, 2014	Dec. 31, 2013
Estimated fair value | Pension benefits
Fair value measurements on a recurring basis
Total assets	$ 1,291	$ 1,233
Cash, receivables and payables, net	(25)	(46)
Fair value of plan assets	1,266	1,187
Estimated fair value | Pension benefits | Equity securities
Fair value measurements on a recurring basis
Fair value of plan assets	649	672
Estimated fair value | Pension benefits | Equity index funds
Fair value measurements on a recurring basis
Fair value of plan assets	132	127
Estimated fair value | Pension benefits | Fixed income securities
Fair value measurements on a recurring basis
Fair value of plan assets	428	350
Estimated fair value | Pension benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	82	84
Estimated fair value | Other benefits
Fair value measurements on a recurring basis
Total assets	181	180
Cash, receivables and payables, net	(1)	(1)
Fair value of plan assets	180	179
Estimated fair value | Other benefits | Equity securities
Fair value measurements on a recurring basis
Fair value of plan assets	99	102
Estimated fair value | Other benefits | Equity index funds
Fair value measurements on a recurring basis
Fair value of plan assets	19	19
Estimated fair value | Other benefits | Fixed income securities
Fair value measurements on a recurring basis
Fair value of plan assets	49	46
Estimated fair value | Other benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	14	13
Level 1 | Pension benefits
Fair value measurements on a recurring basis
Total assets	903	921
Level 1 | Pension benefits | Equity securities
Fair value measurements on a recurring basis
Fair value of plan assets	649	672
Level 1 | Pension benefits | Equity index funds
Fair value measurements on a recurring basis
Fair value of plan assets	132	127
Level 1 | Pension benefits | Fixed income securities
Fair value measurements on a recurring basis
Fair value of plan assets	121	122
Level 1 | Pension benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	1	0
Level 1 | Other benefits
Fair value measurements on a recurring basis
Total assets	164	161
Level 1 | Other benefits | Equity securities
Fair value measurements on a recurring basis
Fair value of plan assets	99	102
Level 1 | Other benefits | Equity index funds
Fair value measurements on a recurring basis
Fair value of plan assets	19	19
Level 1 | Other benefits | Fixed income securities
Fair value measurements on a recurring basis
Fair value of plan assets	43	40
Level 1 | Other benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	3
Level 2 | Pension benefits
Fair value measurements on a recurring basis
Total assets	388	311
Level 2 | Pension benefits | Fixed income securities
Fair value measurements on a recurring basis
Fair value of plan assets	307	228
Level 2 | Pension benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	81	83
Level 2 | Other benefits
Fair value measurements on a recurring basis
Total assets	17	19
Level 2 | Other benefits | Fixed income securities
Fair value measurements on a recurring basis
Fair value of plan assets	6	6
Level 2 | Other benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	11	13
Level 3 | Pension benefits
Fair value measurements on a recurring basis
Total assets	0	1
Level 3 | Pension benefits | Pooled and mutual funds and other
Fair value measurements on a recurring basis
Fair value of plan assets	$ 0	$ 1