Secured Debt Agreements, Net - Credit Facilities (Parenthetical) (Detail) - Credit Facilities [Member]	Sep. 30, 2017 USD ($)	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 EUR (€)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 EUR (€)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 7,137,810,000			$ 5,479,180,000
Maximum Facility Size	7,137,810,000			5,479,180,000
Maximum Facility Size	7,137,810,000			5,479,180,000
Maximum Facility Size	7,137,810,000			5,479,180,000
Societe Generale [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	472,560,000		€ 400,000,000	420,680,000		€ 400,000,000
Maximum Facility Size	472,560,000		400,000,000	420,680,000		400,000,000
Maximum Facility Size	472,560,000		400,000,000	420,680,000		400,000,000
Maximum Facility Size	472,560,000		400,000,000	420,680,000		€ 400,000,000
Citibank [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	795,350,000			500,000,000
Maximum Facility Size	795,350,000			500,000,000
Maximum Facility Size	795,350,000			500,000,000
Maximum Facility Size	795,350,000			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	295,400,000		250,000,000
Maximum Facility Size	295,400,000		250,000,000
Maximum Facility Size	295,400,000		250,000,000
Maximum Facility Size	295,400,000		€ 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	669,900,000	£ 500,000,000		308,500,000	£ 250,000,000
Maximum Facility Size	669,900,000	500,000,000		308,500,000	250,000,000
Maximum Facility Size	669,900,000	500,000,000		308,500,000	250,000,000
Maximum Facility Size	669,900,000	£ 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