Goodwill, Intangible Assets and Deferred Charge - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill	$ 73,071,000	$ 73,071,000	$ 60,071,000
Impairment of goodwill	0
Amortization of intangible assets	12,200,000	$ 5,800,000	$ 21,200,000
Non-Life Run-Off Segment
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill	21,200,000
Atrium
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill	38,900,000
StarStone Segment
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill	13,000,000
Asset impairment charges	$ 4,000,000
Minimum
Schedule Of Goodwill And Intangible Assets [Line Items]
Finite lived intangible assets amortization period (years)	4 years
Maximum
Schedule Of Goodwill And Intangible Assets [Line Items]
Finite lived intangible assets amortization period (years)	15 years