Investments - Schedule of investments, categorized by the level of the fair value hierarchy (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Assets
Equity securities	$ 1,157,075	$ 857,817
Private common equity securities	5,288	96,294
Total equities	1,162,363	954,111
Asset-backed securities	371,458	325,533
Bank debts	11,829	8,017
Corporate bonds	104,864	86,749
Municipal bonds	19,431	10,486
Sovereign debt	43,871	10,639
Total debt securities	551,453	441,424
Investments in limited partnerships	72,263	34,578
Options	2,783	13,069
Rights and warrants	297	1
Trade claims	16,066	17,681
Catastrophe bond	2,003
Total other investments	93,412	65,329
Derivative assets, at fair value	22,516	39,045
Total assets	1,829,744	1,499,909
Liabilities
Equity securities	4,008	5,207
Sovereign debt	33,213	37,592
Corporate bonds	4,699	3,372
Options	5,074	9,885
Total securities sold, not yet purchased	46,994	56,056
Derivative liabilities (embedded)	5,538	4,430
Derivative liabilities, at fair value	10,528	8,819
Total liabilities	63,060	69,305
Quoted prices in active markets (Level 1)
Assets
Equity securities	1,118,096	839,903
Private common equity securities	0	0
Total equities	1,118,096	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	2,619	6,284
Rights and warrants	297	1
Trade claims	0	0
Catastrophe bond	0
Total other investments	2,916	6,285
Derivative assets, at fair value	0	321
Total assets	1,121,012	846,509
Liabilities
Equity securities	4,008	5,207
Sovereign debt	0	0
Corporate bonds	0	0
Options	3,614	4,714
Total securities sold, not yet purchased	7,622	9,921
Derivative liabilities (embedded)	0	0
Derivative liabilities, at fair value	532	441
Total liabilities	8,154	10,362
Significant other observable inputs (Level 2)
Assets
Equity securities	38,979	17,914
Private common equity securities	2,988	94,282
Total equities	41,967	112,196
Asset-backed securities	369,017	325,133
Bank debts	11,829	8,017
Corporate bonds	99,711	82,139
Municipal bonds	19,431	10,486
Sovereign debt	43,841	10,639
Total debt securities	543,829	436,414
Investments in limited partnerships	66,492	29,286
Options	164	6,785
Rights and warrants	0	0
Trade claims	16,066	17,681
Catastrophe bond	2,003
Total other investments	84,725	53,752
Derivative assets, at fair value	22,516	38,724
Total assets	693,037	641,086
Liabilities
Equity securities	0	0
Sovereign debt	33,213	37,592
Corporate bonds	4,699	3,372
Options	1,460	5,171
Total securities sold, not yet purchased	39,372	46,135
Derivative liabilities (embedded)	0	0
Derivative liabilities, at fair value	9,996	8,378
Total liabilities	49,368	54,513
Significant unobservable inputs (Level 3)
Assets
Equity securities	0	0
Private common equity securities	2,300	2,012
Total equities	2,300	2,012
Asset-backed securities	2,441	400
Bank debts	0	0
Corporate bonds	5,153	4,610
Municipal bonds	0	0
Sovereign debt	30	0
Total debt securities	7,624	5,010
Investments in limited partnerships	5,771	5,292
Options	0	0
Rights and warrants	0	0
Trade claims	0	0
Catastrophe bond	0
Total other investments	5,771	5,292
Derivative assets, at fair value	0	0
Total assets	15,695	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)	5,538	4,430
Derivative liabilities, at fair value	0	0
Total liabilities	$ 5,538	$ 4,430