Secured Debt Agreements - Revolving Repurchase Facilities (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 GBP (£)
Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 4,804,267,000		$ 3,377,205,000
Wells Fargo [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,172,000,000		1,000,000,000
Wells Fargo [Member] | Asset Specific Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 172,000,000
Facility repurchase date	Jul. 30, 2016
JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 787,047,000		488,155,000
JP Morgan [Member] | Asset Specific Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 262,500,000
Facility repurchase date	Jan. 09, 2018
JP Morgan [Member] | Pledged Assets Repayment Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 47,800,000	£ 32,300,000
Citibank [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	537,500,000		500,000,000
Citibank [Member] | Asset Specific Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 37,500,000
Facility repurchase date	Oct. 09, 2017
Morgan Stanley [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 370,400,000	250,000,000	389,050,000	£ 250,000,000
Societe Generale [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	437,320,000	400,000,000
$250.0 Million Facility [Member] | JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	250,000,000		250,000,000
UK Pound Sterling153.0 Million ($226.7 Million) Facility [Member] | JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	226,700,000	£ 153,000,000	238,200,000	£ 153,000,000
$500.0 Million Facility [Member] | Citibank [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000		500,000,000
$1.0 Billion Facility [Member] | Wells Fargo [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 1,000,000,000		$ 1,000,000,000