Lease Commitments - Additional Information (Detail) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended			1 Months Ended		12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Mar. 28, 2007 7WTC USD ($)	Oct. 20, 2006 7WTC Y	Dec. 31, 2012 7WTC USD ($)	Feb. 06, 2008 Six Floors In Canary Wharf District GBP (£) Y	Dec. 31, 2012 Six Floors In Canary Wharf District USD ($)	Dec. 31, 2012 Computer and other equipment
Lease Commitments And Contingencies [Line Items]
Rent and amortization expenses under operating leases	$ 75.8	$ 73.1	$ 70.9
Expiration of operating lease (in years)	15 years								6 years
Operating lease, period of lease, in years					21		17.5
Renewal options, years					20 years
Additional two floors of lease, years				20 years
Total base rent	815.4			642.0			134.0
Remaining rent obligation						$ 509.2		$ 201.0