Lease Commitments - Lease Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets:
Operating lease assets	$ 1,193	$ 1,385
Finance lease assets	343	397
Total leased assets	1,536	1,782
Liabilities:
Operating lease liabilities, current	453	569
Finance lease liabilities, current	38	51
Operating lease liabilities, non-current	768	854
Finance lease liabilities, non-current	308	347
Total lease liabilities	$ 1,567	$ 1,821