Financial Instruments Owned and Pledged as Collateral at Fair Value, Securities Purchased Under Agreements to Resell and Securities Sold Under Agreements to Repurchase - Summary of Assets, Liabilities and Collateral Received or Pledged (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	$ 19,374	$ 18,535
Securities Purchased Under Agreements to Resell	10,645	12,585
Securities Sold Under Agreements to Repurchase	(30,027)	(31,150)
Asset (Liability) Balance [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	19,374	18,535
Securities Purchased Under Agreements to Resell	10,645	12,585
Total Assets	30,019	31,120
Securities Sold Under Agreements to Repurchase	(30,027)	(31,150)
Market Value of Collateral Received or (Pledged) [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Securities Purchased Under Agreements to Resell	10,643	12,601
Securities Sold Under Agreements to Repurchase	$ (30,020)	$ (31,155)