Secured Debt Agreements, Net - Revolving Repurchase Facilities (Parenthetical) (Detail)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)
Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 5,479,180,000			$ 4,332,267,000
Revolving Repurchase Facilities [Member] | Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	308,500,000	£ 250,000,000		370,400,000
Revolving Repurchase Facilities [Member] | Societe Generale [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	420,680,000		€ 400,000,000	437,320,000
Revolving Repurchase Facilities [Member] | JP Morgan [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			524,547,000
Revolving Repurchase Facilities [Member] | JP Morgan [Member] | $250.0 Million Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 500,000,000			250,000,000
Revolving Repurchase Facilities [Member] | JP Morgan [Member] | UK Pound Sterling 153.0 Million ($226.7 Million) Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size				226,700,000	£ 153,000,000
Pledged Assets Repayment Revolving Repurchase Facilities [Member] | JP Morgan [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size				$ 47,800,000	£ 32,300,000