Commitments and Contingencies - Employee and Office Lease Termination Costs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Expenses [Line Items]
Termination costs, ending balance	$ 13.6	$ 15.2	$ 23.1
Termination Costs [Member]
Expenses [Line Items]
Termination costs, ending balance		15.2	23.1
Termination costs, beginning balance	15.2	23.1	29.6
Compensation expense	5.4	2.8	6.8
Contract termination costs	0.5	1.7	1.7
Costs paid or settled	$ (7.5)	$ (12.4)	$ (15.0)