Variable Interest Entities (Consolidated Variable Interest Entity Asset and Liability Information) (Details) - Derivative Instruments Not Designated and Not Qualifying as Hedging Instruments [Member] $ in Millions	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Disclosure Of Variable Interest Entities Assets And Liabilities [Line Items]
Number of Instruments, Assets | security	1	1
Notional Amounts, Assets | $	$ 574	$ 573
Total Return Swap [Member]
Disclosure Of Variable Interest Entities Assets And Liabilities [Line Items]
Number of Instruments, Assets | security	1	1
Notional Amounts, Assets | $	$ 574	$ 573