Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Commitment to purchase alternative investment		$ 50,000	$ 40,000	$ 50,000
Future Funding Commitments	$ 45,164	$ 57,714
European Non-Performing Loan Fund, LP
Commitments and Contingencies [Line Items]
Funded commitment amount	35,800
Private Middle Market Loans, LLC
Commitments and Contingencies [Line Items]
Funded commitment amount	40,000
Distressed Debt Fund, LP
Commitments and Contingencies [Line Items]
Funded commitment amount	24,200
Unfunded Commitments | European Non-Performing Loan Fund, LP
Commitments and Contingencies [Line Items]
Future Funding Commitments	14,200
Unfunded Commitments | Private Middle Market Loans, LLC
Commitments and Contingencies [Line Items]
Future Funding Commitments	5,200
Unfunded Commitments | Distressed Debt Fund, LP
Commitments and Contingencies [Line Items]
Future Funding Commitments	$ 25,800