Fair Value Disclosures - Narrative (Details) - Level 3 [Member] - USD ($) $ in Billions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets and Liabilities, Fair Value Disclosure	$ 6.0	$ 5.5
Fair Value, Assets and Liabilities Fair Value as Percent of Total Assets	32.00%	31.00%
Fair Value, Assets and Liabilities, Fair Value as Percent of Total Financial Instruments Measured at Fair Value	72.00%	71.00%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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