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Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016
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Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | $ 4,344.6 | $ 4,535.7
Total as percent of investable assets(1) | 48.90% | 49.30%
Letters of credit / guarantees |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | $ 689.9 | $ 672.1
Affiliated transactions |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | 1,311.1 | 1,482.8
Third party |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |
Total restricted assets | $ 2,343.6 | $ 2,380.8

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