Segment Information - Schedule of Earned Premiums by Geographic Location (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 1,303.8	$ 1,186.5	$ 1,082.0
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	101.7	120.4	101.5
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	38.7	26.6	0
Malta [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	1.8	0.3	0
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	401.7	320.8	259.3
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total earned premiums	$ 759.9	$ 718.4	$ 721.2