Investments - Schedule of investments, categorized by the level of the fair value hierarchy (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets
Equity securities	$ 987,295	$ 857,817
Private common equity securities	129,259	96,294
Total equities	1,116,554	954,111
Asset-backed securities	371,450	325,533
Bank debts	832	8,017
Corporate bonds	130,225	86,749
Municipal bonds	28,025	10,486
Sovereign debt	28,116	10,639
Total debt securities	558,648	441,424
Investments in limited partnerships	48,070	34,578
Options	6,607	13,069
Rights and warrants	2	1
Trade claims	22,708	17,681
Total other investments	77,387	65,329
Derivative assets, at fair value	28,134	39,045
Total assets	1,780,723	1,499,909
Liabilities
Equity securities	7,182	5,207
Sovereign debt	36,216	37,592
Corporate bonds	4,624	3,372
Options	5,936	9,885
Total securities sold, not yet purchased	53,958	56,056
Derivative liabilities (embedded)	5,356	4,430
Derivative liabilities, at fair value	7,569	8,819
Total liabilities	66,883	69,305
Quoted prices in active markets (Level 1)
Assets
Equity securities	970,291	839,903
Private common equity securities	0	0
Total equities	970,291	839,903
Asset-backed securities	0	0
Bank debts	0	0
Corporate bonds	0	0
Municipal bonds	0	0
Sovereign debt	0	0
Total debt securities	0	0
Investments in limited partnerships	0	0
Options	2,405	6,284
Rights and warrants	2	1
Trade claims	0	0
Total other investments	2,407	6,285
Derivative assets, at fair value	42	321
Total assets	972,740	846,509
Liabilities
Equity securities	7,182	5,207
Sovereign debt	0	0
Corporate bonds	0	0
Options	3,079	4,714
Total securities sold, not yet purchased	10,261	9,921
Derivative liabilities (embedded)	0	0
Derivative liabilities, at fair value	0	441
Total liabilities	10,261	10,362
Significant other observable inputs (Level 2)
Assets
Equity securities	17,004	17,914
Private common equity securities	127,247	94,282
Total equities	144,251	112,196
Asset-backed securities	367,898	325,133
Bank debts	832	8,017
Corporate bonds	125,444	82,139
Municipal bonds	28,025	10,486
Sovereign debt	28,116	10,639
Total debt securities	550,315	436,414
Investments in limited partnerships	42,962	29,286
Options	4,202	6,785
Rights and warrants	0	0
Trade claims	22,708	17,681
Total other investments	69,872	53,752
Derivative assets, at fair value	28,092	38,724
Total assets	792,530	641,086
Liabilities
Equity securities	0	0
Sovereign debt	36,216	37,592
Corporate bonds	4,624	3,372
Options	2,857	5,171
Total securities sold, not yet purchased	43,697	46,135
Derivative liabilities (embedded)	0	0
Derivative liabilities, at fair value	7,569	8,378
Total liabilities	51,266	54,513
Significant unobservable inputs (Level 3)
Assets
Equity securities	0	0
Private common equity securities	2,012	2,012
Total equities	2,012	2,012
Asset-backed securities	3,552	400
Bank debts	0	0
Corporate bonds	4,781	4,610
Municipal bonds	0	0
Sovereign debt	0	0
Total debt securities	8,333	5,010
Investments in limited partnerships	5,108	5,292
Options	0	0
Rights and warrants	0	0
Trade claims	0	0
Total other investments	5,108	5,292
Derivative assets, at fair value	0	0
Total assets	15,453	12,314
Liabilities
Equity securities	0	0
Sovereign debt	0	0
Corporate bonds	0	0
Options	0	0
Total securities sold, not yet purchased	0	0
Derivative liabilities (embedded)	5,356	4,430
Derivative liabilities, at fair value	0	0
Total liabilities	$ 5,356	$ 4,430