Segment Information - Summary of Company's Assets by Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 11,832,132	$ 9,936,885
Operating Segments | Non-Life Run-Off
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	7,629,184	5,936,187
Operating Segments | Atrium
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	555,621	598,037
Operating Segments | Torus Insurance Holdings Limited
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	2,778,275	2,876,734
Operating Segments | Life and Annuities
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,734,945	1,344,593
Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 865,893	$ 818,666