Variable Interest Entities (Consolidated Variable Interest Entity Asset and Liability Information) (Details) $ in Millions	Mar. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Disclosure Of Variable Interest Entities Assets And Liabilities [Line Items]
Number of Instruments, Assets | item	1	1
Notional Amounts, Assets	$ 562	$ 573
Total Return Swap [Member]
Disclosure Of Variable Interest Entities Assets And Liabilities [Line Items]
Number of Instruments, Assets | item	1	1
Notional Amounts, Assets	$ 562	$ 573
Carrying Value, Assets