Employee benefit plans - Plan assets by asset category segregated by level (Details) - Pensions Plans - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee benefit plans
Total assets at fair value	$ 429	$ 478	$ 437
Level 1
Employee benefit plans
Total assets at fair value	35	40
Level 2
Employee benefit plans
Total assets at fair value	393	437
Level 3
Employee benefit plans
Total assets at fair value	1	1
Cash and cash equivalent
Employee benefit plans
Total assets at fair value	5	6
Cash and cash equivalent | Level 1
Employee benefit plans
Total assets at fair value	5	6
U.S. equity securities
Employee benefit plans
Total assets at fair value	126	147
U.S. equity securities | Level 1
Employee benefit plans
Total assets at fair value	11	14
U.S. equity securities | Level 2
Employee benefit plans
Total assets at fair value	115	133
International equity securities
Employee benefit plans
Total assets at fair value	133	150
International equity securities | Level 1
Employee benefit plans
Total assets at fair value	19	20
International equity securities | Level 2
Employee benefit plans
Total assets at fair value	113	129
International equity securities | Level 3
Employee benefit plans
Total assets at fair value	1	1
Fixed income
Employee benefit plans
Total assets at fair value	158	167
Fixed income | Level 2
Employee benefit plans
Total assets at fair value	158	167
Other
Employee benefit plans
Total assets at fair value	7	8
Other | Level 2
Employee benefit plans
Total assets at fair value	$ 7	$ 8