Dividend Restrictions and Statutory Requirements - Summary of Statutory Capital and Surplus (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Bermuda
Statutory Accounting Practices [Line Items]
Minimum Required Statutory Capital and Surplus	$ 510,773,000	$ 315,870,000
Actual Statutory Capital and Surplus	1,767,172,000	1,509,433,000
Statutory Income	147,883,000	41,750,000	$ 103,852,000
United Kingdom
Statutory Accounting Practices [Line Items]
Minimum Required Statutory Capital and Surplus	61,819,000	76,475,000
Actual Statutory Capital and Surplus	629,208,000	519,227,000
Statutory Income	113,296,000	107,030,000	76,685,000
Australia
Statutory Accounting Practices [Line Items]
Minimum Required Statutory Capital and Surplus	44,921,000	63,174,000
Actual Statutory Capital and Surplus	74,080,000	68,840,000
Statutory Income	10,789,000	6,271,000	19,131,000
United States
Statutory Accounting Practices [Line Items]
Minimum Required Statutory Capital and Surplus	147,538,000	154,732,000
Actual Statutory Capital and Surplus	756,543,000	822,336,000
Statutory Income	14,964,000	91,576,000	4,725,000
Europe
Statutory Accounting Practices [Line Items]
Minimum Required Statutory Capital and Surplus	44,126,000	29,946,000
Actual Statutory Capital and Surplus	211,458,000	182,573,000
Statutory Income	$ 1,856,000	$ 11,959,000	$ 11,035,000