Secured Debt Agreements, Net - Credit Facilities (Parenthetical) (Detail)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Jun. 30, 2018 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 GBP (£)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 660,400,000		£ 500,000,000	$ 675,700,000		£ 500,000,000
Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	8,327,710,000			7,155,975,000
Citibank [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	750,000,000			800,125,000
Citibank [Member] | Credit Facilities [Member] | UK Pound Sterling 250.0 Million ($300.0 Million) Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size				300,100,000	€ 250,000,000
Citibank [Member] | Credit Facilities [Member] | $500.0 Million Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size				500,000,000
Societe Generale [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 467,360,000	€ 400,000,000		$ 480,200,000	€ 400,000,000