Investments - Financial Instrument Obligations (Details) (Financial instrument liabilities, USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	$ 19,415	$ 19,467
Estimated Fair Value	21,308	21,311
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	675	666
Estimated Fair Value	744	621
Market indices
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	18,685	18,685
Estimated Fair Value	20,540	20,451
Covered call options
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	55	7
Estimated Fair Value	24	4
Naked put options
Schedule of Available-for-sale Securities [Line Items]
Initial Obligation	0	109
Estimated Fair Value	$ 0	$ 235