Income Tax Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Rate	Dec. 31, 2013	Dec. 31, 2012
Federal Income Tax Note
2013 UK Tax Rate	23.00%
2014 UK Tax Rate	21.00%
2015 UK Tax Rate	20.00%
Income Tax Reconciliation, Change in Enacted Tax Rate		$ 6,300,000	$ 9,300,000
Excess of Financial Reporting Carrying Amount Over the Tax Basis of Investments in Foreign Subsidiaries, Provision Not Practicable	1,000,000,000
Liability for unrecognized tax benefits	10,400,000	10,200,000	15,000,000
Unrecognized Tax Benefits, Interest on Income Taxes Expense	200,000	(1,100,000)	(10,400,000)
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	1,000,000	800,000
Tax Adjustments, Settlements, and Unusual Provisions	17,500,000		11,000,000
Impact of Reinstatement of Exemption of Active Finance Income	900,000
Operating Loss Carryforwards	0	0
Income Taxes Paid, Net	155,700,000	398,100,000	185,000,000
Maximum
Federal Income Tax Note
Liability (Refund) Adjustment from Settlement with Taxing Authority	$ 19,300,000