Transactions with Affiliates - Schedule of Reinsurance Transactions on Income Statements for Pooling Arrangement (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Written premiums:
Ceded	$ (815.5)	$ (823.4)	$ (858.2)
Assumed	1,293.3	1,273.5	1,194.2
Premiums Written, Net	477.8	450.1	336.0
Earned premiums:
Ceded	(819.9)	(832.9)	(861.7)
Assumed	1,291.9	1,270.5	1,074.1
Premiums Earned, Net	472.0	437.6	212.4
Losses and loss expenses incurred:
Ceded	(560.3)	(570.9)	(523.6)
Assumed	944.1	864.2	774.4
Losses and loss expenses incurred, Net	383.8	293.3	250.8
Acquisition and Operating Expenses [Abstract]
Ceded	(148.5)	(183.9)	(215.4)
Assumed	434.3	432.5	372.3
Acquisition and Operating Expenses, Net	$ 285.8	$ 248.6	$ 156.9