Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
	Sep. 20, 2017	Jun. 30, 2017	Dec. 31, 2017	Jun. 30, 2018	Jun. 14, 2018
Commitment And Contingencies [Line Items]
Non-payment of severance under employment contract					$ 4,547
Outstanding funding commitment			$ 1,390,188	$ 934,389
Lending Arrangements
Commitment And Contingencies [Line Items]
Outstanding funding commitment				10,506
Note B
Commitment And Contingencies [Line Items]
Payment to purchase mortgage loan	$ 18,500
Mortgage loan charged off			$ 18,500
Mortgage loan principal to be received				$ 18,500
Note A
Commitment And Contingencies [Line Items]
Payment to purchase mortgage loan	$ 190,000
Former Employee
Commitment And Contingencies [Line Items]
Principle to settle former employee		$ 175