Transactions with Related Parties - Additional Information (Detail) € in Billions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan shares	Mar. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan shares	Mar. 31, 2018 EUR (€) shares
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 15,492,000		$ 14,284,000
Common stock value	1,082,000		1,079,000
Non-cash expenses	6,976,000	$ 5,907,000
Loan face amount	$ 11,140,975,000		$ 10,108,226,000
Number of Loans | SecurityLoan	106		110
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%
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	1,657,212		1,484,175	1,657,212
Restricted shares, vesting period	3 years
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date	Dec. 19, 2018
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 each anniversary thereafter unless earlier terminated.
Management Agreement renewal term, period	1 year
Accrued management and incentive fees payable	$ 15,500,000		$ 14,300,000
Management fees paid to Manager	14,300,000	12,800,000
Expenses reimbursed to Manager	$ 190,000	$ 93,000
Number of Loans | SecurityLoan	0	2
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
Manager [Member] | Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount	$ 330,000,000
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	804,483			804,483
Common stock value	$ 25,000,000
Non-cash expenses	$ 3,200,000	2,900,000
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	$ 141,000	$ 88,000
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%