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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Premiums Written And Earned [Abstract]      
Written, Direct $ 37.7 $ 44.6 $ 57.6
Written, Assumed 0.1 1.0 4.7
Written, Ceded (37.8) (45.6) (62.3)
Earned, Direct 40.7 59.1 53.2
Earned, Assumed 0.9 1.9 4.6
Earned, Ceded $ (41.6) $ (61.0) $ (57.8)