Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Commitment to purchase alternative investment			$ 50,000	$ 50,000
Future Funding Commitments	$ 43,215	$ 34,714
European Non-Performing Loan Fund, LP
Commitments and Contingencies [Line Items]
Funded commitment amount	35,800
Distressed Debt Fund, LP
Commitments and Contingencies [Line Items]
Funded commitment amount	21,000
Unfunded Commitments | European Non-Performing Loan Fund, LP
Commitments and Contingencies [Line Items]
Future Funding Commitments	14,200
Unfunded Commitments | Distressed Debt Fund, LP
Commitments and Contingencies [Line Items]
Future Funding Commitments	$ 29,000