Transactions with Related Parties - Additional Information (Detail) € in Billions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) SecurityLoan shares	Jun. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan shares	Mar. 31, 2018 USD ($)	Mar. 31, 2018 EUR (€)
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 22,388,000		$ 22,388,000		$ 14,284,000
Common stock value	1,114,000		1,114,000		1,079,000
Non-cash expenses			13,754,000	$ 11,866,000
Loan face amount	12,303,596,000		$ 12,303,596,000		$ 10,108,226,000
Number of Loans | SecurityLoan			107		110
Fully subscribed offering costs	$ 474,600,000		$ 474,600,000
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount | €							€ 7.3
Senior term facility | €							€ 1.0
Minority participation in senior term facility						14.00%	14.00%
Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred		$ 79,000		167,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	1,518,667		1,518,667		1,484,175
Restricted shares, vesting period	3 years
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date			Dec. 19, 2017
Management Agreement renewal term, description			The current term of which expires on December 19, 2018, and will be automatically renewed for a one-year term upon such date and each anniversary thereafter unless earlier terminated.
Management Agreement renewal term, period			1 year
Accrued management and incentive fees payable	$ 22,400,000		$ 22,400,000		$ 14,300,000
Management fees paid to Manager	15,500,000	12,900,000	29,800,000	25,700,000
Expenses reimbursed to Manager	215,000	174,000	$ 405,000	$ 266,000
Number of Loans | SecurityLoan			2	3
Expenses incurred on insurance agency transaction of affiliate with loan borrower			$ 0	$ 0
Income received on insurance agency transaction of affiliate with loan borrower			0	0
Securitized debt obligations issued, aggregate amount	72,900,000		72,900,000
Manager [Member] | Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount	$ 1,000,000,000		$ 1,000,000,000			$ 330,000,000
Manager [Member] | Senior Loan [Member] | Partially Owned Properties [Member]
Related Party Transaction [Line Items]
Ownership interest in joint venture	50.00%		50.00%
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	739,908		739,908
Common stock value	$ 23,100,000		$ 23,100,000
Non-cash expenses	$ 3,200,000	$ 2,900,000	6,400,000	5,800,000
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member] | Stock Incentive Current Plan [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period	3 years
Affiliates of Manager [Member]
Related Party Transaction [Line Items]
Special servicing fees			0	$ 0
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 153,000		$ 293,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] | Senior Term Facility [Member]
Related Party Transaction [Line Items]
Ownership interest in joint venture						51.00%	51.00%