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Fair Value Measurements - Transfers between Level 1 and Level 2 (Details) $ in Millions | Dec. 31, 2018 USD ($) stock | Dec. 31, 2017 USD ($) stock | Dec. 31, 2016 USD ($) stock
--- | --- | --- | ---
Perpetual preferred stocks |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Number of stocks transferred from Level 2 to Level 1 | stock | 2 | 4 | 6
Fair value of assets transferred from Level 2 to Level 1 | $ | $ 5 | $ 23 | $ 35
Number of stocks transferred from Level 1 to Level 2 | stock | 2 | 2 | 7
Fair value of assets transferred from Level 1 to Level 2 | $ | $ 6 | $ 11 | $ 28
Common stocks |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Number of stocks transferred from Level 2 to Level 1 | stock | 0 | 0 | 3
Fair value of assets transferred from Level 2 to Level 1 | $ | $ 0 | $ 0 | $ 0
Number of stocks transferred from Level 1 to Level 2 | stock | 0 | 1 | 2
Fair value of assets transferred from Level 1 to Level 2 | $ | $ 0 | $ 0 | $ 0

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- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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