Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Line Items]
Income (Loss) before Income Taxes, Domestic	$ 376,700,000	$ 633,800,000	$ 616,000,000
Income (Loss) before Income Taxes, Foreign	380,700,000	298,100,000	$ 239,200,000
State net operating loss carry forward	17,900,000	16,600,000
Valuation allowance	17,900,000	$ 16,600,000
Annual limitation on the use of foreign tax credit carryforwards and minimum tax credit carryforwards by Transatlantic	42,700,000
Interest or penalties accrued for uncertain tax positions	$ 0