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Fair Value Disclosures - Narrative (Details) - Level 3 [Member] - USD ($) $ in Billions | Mar. 31, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets and Liabilities, Fair Value Disclosure | $ 5.9 | $ 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 | 73.00% | 71.00%

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