Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments in securities:
Trading	$ 55,268	$ 59,906
Fair Value, Measurements, Recurring
Investments in securities:
Trading	55,268	59,906
Total financial instruments	55,268	59,906
Level 1 | Fair Value, Measurements, Recurring
Investments in securities:
Trading	55,268	55,389
Total financial instruments	55,268	55,389
Level 2 | Fair Value, Measurements, Recurring
Investments in securities:
Trading		4,517
Total financial instruments		4,517
Level 3 | Fair Value, Measurements, Recurring
Investments in securities:
Trading
Total financial instruments