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Insurance Operations - Summary of Reconciliations of Direct to Net premiums on Written and Earned Basis Related to Wholly Owned Underwriting Enterprise Subsidiary (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Premiums Written and Earned [Abstract]      
Written, Direct $ 60.7 $ 71.8 $ 71.5
Written, Assumed 5.0 5.2 4.4
Written, Ceded (65.7) (77.0) (75.9)
Written, Net 0.0 0.0 0.0
Earned, Direct 60.4 69.6 71.7
Earned, Assumed 4.5 4.9 5.1
Earned, Ceded (64.9) (74.5) (76.8)
Earned, Net $ 0.0 $ 0.0 $ 0.0