Secured Financings - Revolving Repurchase Facilities Outstanding (Parenthetical) (Detail) (Revolving Repurchase Facilities [Member]) In Millions, unless otherwise specified	Dec. 31, 2014 JP Morgan [Member] USD ($)	Dec. 31, 2014 JP Morgan [Member] GBP (£)	Dec. 31, 2014 Maximum [Member] JP Morgan [Member] USD ($)	Dec. 31, 2014 Maximum [Member] Morgan Stanley [Member] USD ($)	Dec. 31, 2014 Maximum [Member] Morgan Stanley [Member] GBP (£)
Line of Credit Facility [Line Items]
Facility size	$ 238.2	£ 153.0	$ 250.0	$ 389.1	£ 250.0