Investments - Carrying Value of Investments in Merchant Banking Funds (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Jul. 31, 2013	Dec. 31, 2012
Investments in and Advances to Affiliates [Line Items]
Investments in merchant banking funds	$ 11,285		$ 16,772
GCP I [Member]
Investments in and Advances to Affiliates [Line Items]
Investments in merchant banking funds	1,732		2,247
GCP II [Member]
Investments in and Advances to Affiliates [Line Items]
Investments in merchant banking funds	7,627		11,173
GCP III [Member]
Investments in and Advances to Affiliates [Line Items]
Investments in merchant banking funds	0	2,000	1,367
Other Merchant Banking Funds
Investments in and Advances to Affiliates [Line Items]
Investments in merchant banking funds	$ 1,926		$ 1,985