Variable Interest Entities ("VIEs") - Summary of Assets and Liabilities of Variable Interest Entities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Variable Interest Entity [Line Items]
Cash and restricted cash	$ 53,864	$ 55,728
Total assets	8,952,978	9,030,441
Other liabilities	45,742	43,950
Total liabilities	3,443,630	3,475,488
Variable Interest Entity [Member]
Variable Interest Entity [Line Items]
Investments, at fair value	17,109	17,136
Investments, at cost	63,542	63,550
Cash and restricted cash	4,823	4,976
Total assets	85,474	85,662
Loans payable to non-controlling interests	43,188	42,195
Other liabilities	102	131
Total liabilities	$ 43,290	$ 42,326