Summary of Significant Accounting Policies (Adoption of Updated Guidance) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment at cost (less accumulated depreciation)	$ 282	$ 326	$ 324
Other assets	1,473	1,445	1,208
Other liabilities	3,325	3,328	$ 3,089
Adjustments Due to Adoption of Topic 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment at cost (less accumulated depreciation)		2
Other assets	$ 75	237
Other liabilities		$ 239