Secured Debt Agreements, Net - Credit Facilities (Parenthetical) (Detail) - Credit Facilities [Member]	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 EUR (€)	Dec. 31, 2016 GBP (£)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 7,155,975,000			$ 5,479,180,000
Maximum Facility Size	7,155,975,000			5,479,180,000
Maximum Facility Size	7,155,975,000			5,479,180,000
Maximum Facility Size	7,155,975,000			5,479,180,000
Societe Generale [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	480,200,000	€ 400,000,000		420,680,000	€ 400,000,000
Maximum Facility Size	480,200,000	400,000,000		420,680,000	400,000,000
Maximum Facility Size	480,200,000	400,000,000		420,680,000	400,000,000
Maximum Facility Size	480,200,000	400,000,000		420,680,000	€ 400,000,000
Citibank [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	800,125,000			500,000,000
Maximum Facility Size	800,125,000			500,000,000
Maximum Facility Size	800,125,000			500,000,000
Maximum Facility Size	800,125,000			500,000,000
Citibank [Member] | $250.0 Million Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Maximum Facility Size	500,000,000
Maximum Facility Size	500,000,000
Maximum Facility Size	500,000,000
Citibank [Member] | $500.0 Million Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	300,100,000	250,000,000
Maximum Facility Size	300,100,000	250,000,000
Maximum Facility Size	300,100,000	250,000,000
Maximum Facility Size	300,100,000	€ 250,000,000
Citibank [Member] | UK Pound Sterling 153.0 Million ($226.7 Million) Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size				500,000,000
Maximum Facility Size				500,000,000
Maximum Facility Size				500,000,000
Maximum Facility Size				500,000,000
Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	675,650,000		£ 500,000,000	308,500,000		£ 250,000,000
Maximum Facility Size	675,650,000		500,000,000	308,500,000		250,000,000
Maximum Facility Size	675,650,000		500,000,000	308,500,000		250,000,000
Maximum Facility Size	675,650,000		£ 500,000,000	308,500,000		£ 250,000,000
JP Morgan [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Maximum Facility Size	500,000,000			500,000,000
Maximum Facility Size	500,000,000			500,000,000
Maximum Facility Size	500,000,000			500,000,000
JP Morgan [Member] | $250.0 Million Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Maximum Facility Size	500,000,000			500,000,000
Maximum Facility Size	500,000,000			500,000,000
Maximum Facility Size	$ 500,000,000			$ 500,000,000