Investments - Schedule of investments, categorized by the level of the fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Equity securities	$ 2,221,130	$ 1,453,221
Private common equity securities	4,794	4,799
Private preferred equity securities	57,126	48,834
Total equities	2,283,050	1,506,854
Asset-backed securities	225,499	254,852
Bank debts	14,550	56,896
Corporate bonds	77,086	218,280
U.S. Treasury securities	249,994	327,016
Sovereign debt	102,569	200,913
Financial Instruments, Owned, Other, at Fair Value	5,460
Total debt securities	675,158	1,057,957
Options	4,951	1,024
Rights and warrants	603
Financial Instruments, Owned, Real Estate, at Fair Value	6,831
Trade claims	7,496	9,022
Total other investments	19,881	10,046
Derivative assets, at fair value	73,372	27,432
Total assets	3,051,461	2,602,289
Investments in funds valued at NAV	17,850	72,655
Total assets	3,069,311	2,674,944
Liabilities
Equity securities	364,215	71,457
Corporate bonds	21,699	17,683
Options	8,364	3,528
Total securities sold, not yet purchased	394,278	92,668
Derivative liabilities, at fair value	14,503	16,050
Derivative liabilities (embedded)	171	92
Total liabilities	408,952	108,810
Quoted prices in active markets (Level 1)
Assets
Equity securities	2,200,379	1,450,966
Private common equity securities	0	0
Private preferred equity securities	0	0
Total equities	2,200,379	1,450,966
Asset-backed securities	0	0
Bank debts	0	0
Corporate bonds	0	0
U.S. Treasury securities	0	0
Sovereign debt	0	0
Financial Instruments, Owned, Other, at Fair Value	0
Total debt securities	0	0
Options	1,973	343
Rights and warrants	0
Financial Instruments, Owned, Real Estate, at Fair Value	0
Trade claims	0	0
Total other investments	1,973	343
Derivative assets, at fair value	0	961
Total assets	2,202,352	1,452,270
Liabilities
Equity securities	364,215	71,457
Corporate bonds	0	0
Options	2,668	0
Total securities sold, not yet purchased	366,883	71,457
Derivative liabilities, at fair value	0	1,608
Derivative liabilities (embedded)	0	0
Total liabilities	366,883	73,065
Significant other observable inputs (Level 2)
Assets
Equity securities	20,751	2,255
Private common equity securities	0	0
Private preferred equity securities	0	0
Total equities	20,751	2,255
Asset-backed securities	198,191	237,224
Bank debts	14,550	48,546
Corporate bonds	67,218	209,025
U.S. Treasury securities	249,994	327,016
Sovereign debt	102,569	200,913
Financial Instruments, Owned, Other, at Fair Value	4,747
Total debt securities	637,269	1,022,724
Options	2,978	681
Rights and warrants	168
Financial Instruments, Owned, Real Estate, at Fair Value	0
Trade claims	7,496	9,022
Total other investments	10,642	9,703
Derivative assets, at fair value	73,372	26,471
Total assets	742,034	1,061,153
Liabilities
Equity securities	0	0
Corporate bonds	21,699	17,683
Options	5,696	3,528
Total securities sold, not yet purchased	27,395	21,211
Derivative liabilities, at fair value	12,418	13,116
Derivative liabilities (embedded)	0	0
Total liabilities	39,813	34,327
Significant unobservable inputs (Level 3)
Assets
Equity securities	0	0
Private common equity securities	4,794	4,799
Private preferred equity securities	57,126	48,834
Total equities	61,920	53,633
Asset-backed securities	27,308	17,628
Bank debts	0	8,350
Corporate bonds	9,868	9,255
U.S. Treasury securities	0	0
Sovereign debt	0	0
Financial Instruments, Owned, Other, at Fair Value	713
Total debt securities	37,889	35,233
Options	0	0
Rights and warrants	435
Financial Instruments, Owned, Real Estate, at Fair Value	6,831
Trade claims	0	0
Total other investments	7,266	0
Derivative assets, at fair value	0	0
Total assets	107,075	88,866
Liabilities
Equity securities	0	0
Corporate bonds	0	0
Options	0	0
Total securities sold, not yet purchased	0	0
Derivative liabilities, at fair value	2,085	1,326
Derivative liabilities (embedded)	171	92
Total liabilities	$ 2,256	$ 1,418