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Deferred Policy Acquisition Costs (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]                              
Balance at the beginning of the period       $ 358.4       $ 361.1         $ 358.4 $ 361.1  
Acquisition costs deferred                         336.4 526.2  
Amortization of deferred policy acquisition costs $ (85.1) $ (105.4) $ (96.3) $ (113.7) $ (141.1) $ (130.9) $ (126.7) $ (130.2) $ (118.2) $ (132.0) $ (114.1) $ (119.3) (400.5) (528.9) $ (483.6)
Balance at the end of the period $ 294.3       $ 358.4       $ 361.1       $ 294.3 $ 358.4 $ 361.1