Secured Debt Agreements, Net - Credit Facilities (Parenthetical) (Detail)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 EUR (€)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 700,900,000	£ 500,000,000		$ 675,700,000	£ 500,000,000
Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	8,392,900,000			7,155,975,000
Societe Generale [Member] | Credit Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	492,000,000		€ 400,000,000	480,200,000		€ 400,000,000
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] | $500.0 Million Facility [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size				500,000,000
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