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Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Restricted assets | $ 4,525.9 | $ 4,638.8
Total as percent of investable assets(2) | 56.00% | 53.40%
Securities and cash as collateral secured letters of credit | $ 840.3 | $ 658.5
Letters of credit / guarantees |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Restricted assets | 840.3 | 658.5
Affiliated transactions |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Restricted assets | 1,154.5 | 1,455.0
Third party |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Restricted assets | 2,431.1 | 2,425.3
Limited Partner | Real estate funds |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Restricted assets | 13.8 | 100.0
Purchase Commitment | Limited Partner | Real estate funds |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Restricted assets | $ 86.2 | $ 0.0

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