Secured Debt Agreements - Revolving Repurchase Facilities (Parenthetical) (Detail) - Jun. 30, 2015	USD ($)	GBP (£)
JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Facility size	$ 240,800,000	£ 153,000,000
Facility repurchase date	Jan. 09, 2018
JP Morgan [Member] | Revolving Repurchase Facilities [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Facility size	$ 250,000,000
JP Morgan [Member] | Asset-Specific Repurchase Agreements [Member]
Line of Credit Facility [Line Items]
Facility size	262,500,000
Morgan Stanley [Member] | Revolving Repurchase Facilities [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Facility size	$ 393,500,000	£ 250,000,000