Secured Debt Agreements - Revolving Repurchase Facilities (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2016 GBP (£)	Mar. 31, 2016 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)
Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 4,730,915,000			$ 4,804,267,000
JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	762,500,000			787,047,000
JP Morgan [Member] | Asset Specific Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 262,500,000			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			537,500,000
Citibank [Member] | Asset Specific Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 37,500,000			37,500,000
Facility repurchase date	Oct. 09, 2017
Wells Fargo [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 1,172,000,000			1,172,000,000
Wells Fargo [Member] | Asset Specific Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 172,000,000			172,000,000
Facility repurchase date	Jul. 30, 2016
Morgan Stanley [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 359,450,000	£ 250,000,000		370,400,000
Societe Generale [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	453,520,000		€ 400,000,000	437,320,000
$250.0 Million Facility [Member] | JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			250,000,000
UK Pound Sterling 153.0 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
$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