Leases - Disclosures related to periods prior to adoption of new lease standard (Details) £ in Millions, $ in Millions |
12 Months Ended | ||
|---|---|---|---|
|
Dec. 31, 2018
USD ($)
|
Dec. 31, 2017
USD ($)
|
Dec. 31, 2003
GBP (£)
|
|
| Capital and Operating Leases | |||
| Rental expense under operating leases | $ 393 | $ 343 | |
| Accumulated depreciation for capital leases | 54 | ||
| Capital lease asset and obligation | 92 | ||
| Building in United Kingdom | |||
| Capital and Operating Leases | |||
| Capital lease asset and obligation | 43 | £ 34 | |
| Capital lease term (in years) | 22 years | ||
| Capital lease obligations in aggregate | |||
| Capital and Operating Leases | |||
| Capital lease asset and obligation | $ 13 | $ 13 | |
| Capital lease term (in years) | 15 years | ||