Investments - Financial Instrument Obligations (Details) - Financial instrument liabilities - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	$ 19,399	$ 19,467
Estimated Fair Value	21,385	21,311
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	675	666
Estimated Fair Value	868	621
Market indices
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	18,685	18,685
Estimated Fair Value	20,482	20,451
Covered call options
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	39	7
Estimated Fair Value	35	4
Naked put options
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	0	109
Estimated Fair Value	$ 0	$ 235