Income Tax Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Federal Income Tax Note
2015 UK Tax Rate	20.00%
2017 UK Tax Rate	19.00%
2020 UK Tax Rate	18.00%
2013 UK Tax Rate	23.00%
2014 UK Tax Rate	21.00%
Income Tax Benefit from Enacted Tax Rate Changes	$ 6,500,000	$ 6,300,000
Excess of Financial Reporting Carrying Amount Over the Tax Basis of Investments in Foreign Subsidiaries, Provision Not Practicable	1,000,000,000.0
Tax Expense Not Recognized Due To Permanent Differences	200,000,000
Liability for unrecognized tax benefits		10,400,000	$ 10,200,000
Unrecognized Tax Benefits, Interest on Income Taxes Expense	(1,000,000)	200,000	(1,100,000)
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		1,000,000
Tax Adjustments, Settlements, and Unusual Provisions	6,800,000
Operating Loss Carryforwards	0	0
Valuation Allowance on Deferred Tax Assets	1,300,000	0
Income Taxes Paid, Net	$ 189,100,000	$ 155,700,000	$ 398,100,000