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Commitments and Contingencies - Schedule of Company's Restricted Assets (Details) £ in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2017 GBP (£) | Dec. 31, 2016 GBP (£)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |  |  |
Total restricted assets | $ 4,638.8 | $ 4,535.7 |  |
Total as percent of investable assets | 53.40% | 49.30% |  |
Securities and cash as collateral secured letters of credit | $ 658.5 | $ 672.0 | £ 0.0 | £ 7.2
Affiliated transactions |  |  |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |  |  |
Total restricted assets | 1,455.0 | 1,482.8 |  |
Third party |  |  |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |  |  |
Total restricted assets | 2,425.3 | 2,380.8 |  |
Letters of credit / guarantees |  |  |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |  |  |
Total restricted assets | 658.5 | 672.1 |  |
Real estate fund |  |  |  |
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items] |  |  |  |
Total restricted assets | $ 100.0 | $ 0.0 |  |

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