Investments - Financial Instrument Obligations (Details) - Financial instrument liabilities - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	$ 19,473	$ 19,467
Estimated Fair Value	20,171	21,311
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	675	666
Estimated Fair Value	1,024	621
Market indices
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	18,685	18,685
Estimated Fair Value	19,067	20,451
Covered call options
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	113	7
Estimated Fair Value	80	4
Naked put options
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	0	109
Estimated Fair Value	$ 0	$ 235