Transactions with Related Parties - Additional Information (Detail) € in Billions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) SecurityLoan shares	Sep. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan shares	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 EUR (€)	Jun. 30, 2017 USD ($)
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 18,368,000		$ 18,368,000		$ 14,284,000
Common stock value	1,197,000		1,197,000		1,079,000
Non-cash expenses			20,488,000	$ 17,809,000
Loan face amount	$ 12,260,103,000		$ 12,260,103,000		$ 10,108,226,000
Number of Loans | SecurityLoan			112		110
Fully subscribed offering costs						$ 1,000,000,000				$ 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		$ 87,000		254,000
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	1,380,855		1,380,855		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	$ 18,400,000		$ 18,400,000		$ 14,300,000
Management fees paid to Manager	22,400,000	14,400,000	52,200,000	40,100,000
Expenses reimbursed to Manager	$ 167,000	59,000	$ 572,000	$ 325,000
Number of Loans | SecurityLoan			3	5
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						$ 116,100,000				$ 72,900,000
Manager [Member] | Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount							$ 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%
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	675,333		675,333
Common stock value	$ 21,200,000		$ 21,200,000
Non-cash expenses	$ 3,200,000	$ 2,900,000	9,600,000	8,700,000
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	$ 90,000		$ 384,000
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%