Summary of Significant Accounting Policies - Early Adoption (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Net receivables	$ 4,317		$ 4,201	$ 4,133
Other current assets	551		542	224
Other assets	1,539		1,656	1,430
Deferred tax assets			552	669
Accounts payable and accrued liabilities	2,234		2,205	2,083
Other liabilities	1,362		1,334	1,311
Other Accumulated Comprehensive Income			(14)	0
Retained earnings	14,347		13,504	$ 13,140
Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Net receivables	4,317		68
Other current assets	551		318
Other assets	1,539		226
Deferred tax assets	680		(103)
Accounts payable and accrued liabilities	2,234		122
Other liabilities	$ 1,362		23
Other Accumulated Comprehensive Income			0
Retained earnings		$ 364	364
Accounting Standards Update 2016-01
Disaggregation of Revenue [Line Items]
Net receivables			0
Other current assets			0
Other assets			0
Deferred tax assets			0
Accounts payable and accrued liabilities			0
Other liabilities			0
Other Accumulated Comprehensive Income			(14)
Retained earnings			14
Accounting Standards Update 2016-16
Disaggregation of Revenue [Line Items]
Net receivables			0
Other current assets			0
Other assets			0
Deferred tax assets			(14)
Accounts payable and accrued liabilities			0
Other liabilities			0
Other Accumulated Comprehensive Income			0
Retained earnings			$ (14)