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Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | $ 4,574.9 | $ 4,638.8
Total as percent of investable assets(2) | 55.60% | 53.40%
Securities and cash as collateral secured letters of credit | $ 792.7 | $ 658.5
Letters of credit / guarantees |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | 792.7 | 658.5
Affiliated transactions |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | 1,366.1 | 1,455.0
Third party |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | 2,416.1 | 2,425.3
Real estate funds |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | $ 100.0 | $ 100.0

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