Acquisitions - Additional Information (Detail) $ / shares in Units, € in Millions												3 Months Ended								12 Months Ended
	Nov. 13, 2015 USD ($)	Nov. 13, 2015 EUR (€)	May. 05, 2015 USD ($) Company installment	Jan. 27, 2015 USD ($)	Jun. 10, 2014 $ / shares shares	Apr. 01, 2014 USD ($) insurance_vehicle $ / shares shares	Nov. 25, 2013 USD ($)	Sep. 09, 2013 USD ($)	Jul. 08, 2013 USD ($) $ / shares shares	Mar. 31, 2013 USD ($) insurer	Feb. 07, 2013 USD ($)	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) $ / shares	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares	Dec. 31, 2013 USD ($)	May. 08, 2014 USD ($) member	May. 07, 2014	Mar. 30, 2014
Business Acquisition [Line Items]
Net incurred losses and loss adjustment expenses												$ (64,062,000)	$ 32,359,000	$ 65,900,000	$ 70,136,000	$ (56,085,000)	$ 17,533,000	$ 59,749,000	$ (12,051,000)	$ 104,333,000	$ 9,146,000	$ (163,672,000)
Net premiums earned												$ 197,091,000	$ 231,051,000	$ 212,023,000	$ 198,906,000	$ 171,889,000	$ 195,987,000	$ 216,916,000	$ 61,658,000	$ 839,071,000	$ 646,450,000	239,807,000
Common shares, par value (dollars per share) | $ / shares												$ 1				$ 1				$ 1	$ 1
Preferred shares issued | shares												0								0
Investment in entities																							$ 15,400,000
Goodwill recognized												$ 73,071,000				$ 73,071,000				$ 73,071,000	$ 73,071,000	60,071,000
Non-Life Run-Off Segment
Business Acquisition [Line Items]
Net incurred losses and loss adjustment expenses																				$ (270,830,000)	$ (264,711,000)	$ (182,975,000)
Trident
Business Acquisition [Line Items]
Percentage of interest directly owned						40.00%
Trident | Northshore and StarStone Special Holdings
Business Acquisition [Line Items]
Ownership percentage																							40.00%
Interest owned by co-investor (percent)																							39.30%
Trident | Northshore
Business Acquisition [Line Items]
Interest owned by an entity (percent)																							39.32%	40.00%
Trident | StarStone Specialty Holdings
Business Acquisition [Line Items]
Number of members appointed to board of directors | member																							2
Interest owned by an entity (percent)																							39.32%	40.00%	40.00%
Dowling | Northshore and StarStone Special Holdings
Business Acquisition [Line Items]
Interest owned by co-investor (percent)																							1.70%
Enstar Group Limited
Business Acquisition [Line Items]
Common shares, par value (dollars per share) | $ / shares												$ 1				$ 1				$ 1	$ 1
Kenmare
Business Acquisition [Line Items]
Percentage of interest directly owned						60.00%
Kenmare | Northshore and StarStone Special Holdings
Business Acquisition [Line Items]
Ownership percentage																							60.00%
Interest owned by co-investor (percent)																							59.00%
Kenmare | StarStone Specialty Holdings
Business Acquisition [Line Items]
Number of members appointed to board of directors | member																							3
Nationale Suisse Assurance S.A.
Business Acquisition [Line Items]
Acquisition purchase price	$ 35,225,000	€ 32.8
Earned premiums																				$ 0
Net incurred losses and loss adjustment expenses																				0
Net earnings (losses)																				(100,000)
Purchase price for amalgamation	35,225,000
Goodwill recognized	$ 0
Wilton Relife Settlements
Business Acquisition [Line Items]
Acquisition purchase price			$ 173,058,000
Net earnings (losses)																				$ 16,500,000
Number of payment installments | installment			2
Amount of first installment paid			$ 89,100,000
Aggregate cash purchase price			$ 83,900,000
Number of companies acquired | Company			2
Percentage of common stock												100.00%								100.00%
Purchase price for amalgamation			$ 173,058,000
Goodwill recognized			$ 0
Sussex Insurance Company
Business Acquisition [Line Items]
Acquisition purchase price				$ 218,000,000
Aggregate cash purchase price				$ 218,000,000
Percentage of consideration financed through bank loan facility				50.00%
Percentage of consideration paid from cash on hand				50.00%
Increase (decrease) in unearned premiums fair value adjustment				$ (34,600,000)
Purchase price for amalgamation				218,000,000
Goodwill recognized				$ 0
StarStone
Business Acquisition [Line Items]
Acquisition purchase price						$ 656,088,000			$ 646,000,000
Aggregate cash purchase price						41,600,000
Purchase price						656,088,000
Increase (decrease) in unearned premiums fair value adjustment						$ (11,100,000)						$ (500,000)								$ (500,000)
Unearned premiums amortized to acquisition costs																				$ 10,600,000
Number of wholly-owned insurance vehicles | insurance_vehicle						6
Purchase price for amalgamation						$ 643,088,000
Amalgamation agreement date									Jul. 08, 2013
Acquisition purchase price, per share | $ / shares						$ 136.31			$ 132.448										$ 136.31
Average closing price of voting ordinary shares prior to trading period (days)									20 days
Acquisition related transaction expenses						$ 3,600,000
Goodwill recognized						13,000,000
Indefinite lived intangible assets recognized						23,900,000
Other intangible assets recognized						20,000,000
StarStone | Trident
Business Acquisition [Line Items]
Acquisition purchase price						258,400,000
Acquisition related transaction expenses						2,400,000
StarStone | Trident | StarStone Specialty Holdings
Business Acquisition [Line Items]
Interest owned by an entity (percent)												100.00%								100.00%
StarStone | Enstar Group Limited
Business Acquisition [Line Items]
Company contributed purchase price						$ 397,700,000
First Reserve
Business Acquisition [Line Items]
Common shares, par value (dollars per share) | $ / shares					$ 714,015
Ordinary shares, shares issued (shares) | shares									1,501,211
Preferred shares issued | shares									714,015
Ownership percentage of ordinary shares					9.50%
Ownership percentage of outstanding share capital					11.50%
Corsair
Business Acquisition [Line Items]
Ordinary shares, shares issued (shares) | shares					397,115
Atrium
Business Acquisition [Line Items]
Acquisition purchase price							$ 158,000,000
Purchase price							158,000,000
Increase (decrease) in unearned premiums fair value adjustment							(16,700,000)
Unearned premiums amortized to acquisition costs																				$ 700,000	$ 16,000,000
Purchase price for amalgamation							119,152,000
Goodwill recognized							38,848,000
Indefinite lived intangible assets recognized							63,000,000
Other intangible assets recognized							27,000,000
Atrium | Trident | Northshore
Business Acquisition [Line Items]
Interest owned by an entity (percent)												100.00%								100.00%
Atrium | Lloyd's Syndicate
Business Acquisition [Line Items]
Percentage of premium income from license																				25.00%
Atrium | Kenmare
Business Acquisition [Line Items]
Acquisition purchase price							$ 94,800,000
Arden
Business Acquisition [Line Items]
Acquisition purchase price								$ 79,600,000
Purchase price for amalgamation								79,600,000
Arden | Trident | Northshore
Business Acquisition [Line Items]
Interest owned by an entity (percent)												100.00%								100.00%
Arden | Kenmare
Business Acquisition [Line Items]
Acquisition purchase price								$ 47,800,000
Pavonia
Business Acquisition [Line Items]
Acquisition purchase price										$ 155,564,000
Aggregate cash purchase price										155,600,000
Purchase price for amalgamation										$ 155,564,000
Number of insurers | insurer										3
Purchase price financed										$ 55,700,000
SeaBright
Business Acquisition [Line Items]
Acquisition purchase price											$ 252,091,000
Aggregate cash purchase price											252,100,000
Increase (decrease) in unearned premiums fair value adjustment																$ 14,400,000					$ 14,400,000
Purchase price for amalgamation											252,091,000
Purchase price financed											$ 111,000,000
Period of term loan facility (years)											4 years
Non-Life Run-Off Segment
Business Acquisition [Line Items]
Goodwill recognized												$ 21,200,000								$ 21,200,000
Non-Life Run-Off Segment | Sussex Insurance Company
Business Acquisition [Line Items]
Earned premiums																				43,200,000
Net incurred losses and loss adjustment expenses																				44,400,000
Net earnings (losses)																				(42,400,000)
Voting Ordinary Shares | StarStone
Business Acquisition [Line Items]
Common shares, par value (dollars per share) | $ / shares									$ 1.00
Adjustments Due to Amortization | Sussex Insurance Company
Business Acquisition [Line Items]
Increase (decrease) in unearned premiums fair value adjustment												$ (5,100,000)								(5,100,000)
Unearned premiums amortized to acquisition costs																				16,100,000
Net premiums earned																				$ 13,400,000
Ordinary Shares | Voting Ordinary Shares | StarStone
Business Acquisition [Line Items]
Shares issued | shares						1,898,326
Preferred Stock | Share Capital Series B Convertible Participating Non-Voting Perpetual Preferred Stock | StarStone
Business Acquisition [Line Items]
Shares issued | shares						714,015