Investments - Schedule of investments, categorized by the level of the fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Equity securities	$ 1,453,221	$ 1,201,623
Private common equity securities	4,799	5,276
Private preferred equity securities	48,834	24,178
Total equities	1,506,854	1,231,077
Asset-backed securities	254,852	494,723
Bank debts	56,896	9,818
Corporate bonds	218,280	83,190
U.S. Treasury securities	327,016	186,471
Sovereign debt	200,913	260,045
Total debt securities	1,057,957	1,034,247
Options	1,024	8,911
Rights and warrants		416
Trade claims	9,022	8,329
Total other investments	10,046	17,656
Derivative assets, at fair value	27,432	35,337
Total assets	2,602,289	2,318,317
Investments in funds valued at NAV	72,655	34,264
Total assets	2,674,944	2,352,581
Liabilities
Equity securities	71,457	228,009
Sovereign debt		5,856
Corporate bonds	17,683	76,131
Options	3,528	4,357
Total securities sold, not yet purchased	92,668	314,353
Derivative liabilities, at fair value	16,050	15,392
Derivative liabilities (embedded)	92	5,563
Total liabilities	108,810	335,308
Quoted prices in active markets (Level 1)
Assets
Equity securities	1,450,966	1,181,865
Private common equity securities	0	0
Private preferred equity securities	0	0
Total equities	1,450,966	1,181,865
Asset-backed securities	0	0
Bank debts	0	0
Corporate bonds	0	0
U.S. Treasury securities	0	0
Sovereign debt	0	0
Total debt securities	0	0
Options	343	0
Rights and warrants		416
Trade claims	0	0
Total other investments	343	416
Derivative assets, at fair value	961	0
Total assets	1,452,270	1,182,281
Liabilities
Equity securities	71,457	228,009
Sovereign debt		0
Corporate bonds	0	0
Options	0	690
Total securities sold, not yet purchased	71,457	228,699
Derivative liabilities, at fair value	1,608	0
Derivative liabilities (embedded)	0	0
Total liabilities	73,065	228,699
Significant other observable inputs (Level 2)
Assets
Equity securities	2,255	19,758
Private common equity securities	0	919
Private preferred equity securities	0	0
Total equities	2,255	20,677
Asset-backed securities	237,224	492,106
Bank debts	48,546	2,158
Corporate bonds	209,025	79,938
U.S. Treasury securities	327,016	186,471
Sovereign debt	200,913	260,024
Total debt securities	1,022,724	1,020,697
Options	681	8,911
Rights and warrants		0
Trade claims	9,022	8,329
Total other investments	9,703	17,240
Derivative assets, at fair value	26,471	35,337
Total assets	1,061,153	1,093,951
Liabilities
Equity securities	0	0
Sovereign debt		5,856
Corporate bonds	17,683	76,131
Options	3,528	3,667
Total securities sold, not yet purchased	21,211	85,654
Derivative liabilities, at fair value	13,116	14,372
Derivative liabilities (embedded)	0	0
Total liabilities	34,327	100,026
Significant unobservable inputs (Level 3)
Assets
Equity securities	0	0
Private common equity securities	4,799	4,357
Private preferred equity securities	48,834	24,178
Total equities	53,633	28,535
Asset-backed securities	17,628	2,617
Bank debts	8,350	7,660
Corporate bonds	9,255	3,252
U.S. Treasury securities	0	0
Sovereign debt	0	21
Total debt securities	35,233	13,550
Options	0	0
Rights and warrants		0
Trade claims	0	0
Total other investments	0	0
Derivative assets, at fair value	0	0
Total assets	88,866	42,085
Liabilities
Equity securities	0	0
Sovereign debt		0
Corporate bonds	0	0
Options	0	0
Total securities sold, not yet purchased	0	0
Derivative liabilities, at fair value	1,326	1,020
Derivative liabilities (embedded)	92	5,563
Total liabilities	$ 1,418	$ 6,583