Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 846,218	$ 972,932
Available-for-sale securities	3,172,489	3,254,737
Investments	95,407	79,972
Cash equivalents measured at NAV	51,541	25,734
Derivative contracts, Assets		1,086
Total Assets	4,165,655	4,334,461
Financial instruments sold, but not yet purchased, at fair value	598,322	662,852
Derivative contracts, Liabilities	1,805
Total Liabilities	600,127
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,976	12,403
Financial instruments sold, but not yet purchased, at fair value	22,231	13,271
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	122,160	66,881
Available-for-sale securities	5,146	5,067
Financial instruments sold, but not yet purchased, at fair value	28,361	1,000
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	7,843	28,385
Available-for-sale securities	1,723,814	1,592,889
Financial instruments sold, but not yet purchased, at fair value	675
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	232,618	250,783
Available-for-sale securities	557,969	675,311
Financial instruments sold, but not yet purchased, at fair value	197,111	140,100
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	78,510	64,009
Investments	21,586	35,083
Financial instruments sold, but not yet purchased, at fair value	26,680	32,047
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	123,546	137,211
Available-for-sale securities	2,461	24,297
Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9,611	9,983
Financial instruments sold, but not yet purchased, at fair value		955
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	12,621	14,243
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	183	654
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,333	16,771
Investments in Funds and Partnerships Measured at NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	10,684	13,221
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,722	9,266
Financial instruments sold, but not yet purchased, at fair value	208,591	243,570
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	222,722	388,856
Available-for-sale securities	770,729	837,878
Financial instruments sold, but not yet purchased, at fair value	114,673	231,909
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,510	5,155
Available-for-sale securities	6,923	9,758
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	105,447	109,537
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	87,005	71,717
Investments	29,819	43,928
Total Assets	116,824	115,645
Financial instruments sold, but not yet purchased, at fair value	235,459	276,250
Total Liabilities	235,459
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,062	872
Financial instruments sold, but not yet purchased, at fair value	188	633
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	57,221	61,579
Investments	20,526	34,023
Financial instruments sold, but not yet purchased, at fair value	26,680	32,047
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	9,293	9,905
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,722	9,266
Financial instruments sold, but not yet purchased, at fair value	208,591	243,570
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	749,427	891,889
Available-for-sale securities	3,172,489	3,254,737
Investments	6,039	6,483
Derivative contracts, Assets		1,086
Total Assets	3,927,955	4,154,195
Financial instruments sold, but not yet purchased, at fair value	362,863	385,647
Derivative contracts, Liabilities	1,805
Total Liabilities	364,668
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,976	12,403
Financial instruments sold, but not yet purchased, at fair value	22,231	13,271
Level 2 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	122,160	66,881
Available-for-sale securities	5,146	5,067
Financial instruments sold, but not yet purchased, at fair value	28,361	1,000
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	7,668	28,210
Available-for-sale securities	1,723,814	1,592,889
Financial instruments sold, but not yet purchased, at fair value	675
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	230,556	249,911
Available-for-sale securities	557,969	675,311
Financial instruments sold, but not yet purchased, at fair value	196,923	139,467
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	21,289	2,430
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	123,546	137,211
Available-for-sale securities	2,461	24,297
Level 2 [Member] | Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		832
Level 2 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		470
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	6,039	6,013
Level 2 [Member] | Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	222,722	388,856
Available-for-sale securities	770,729	837,878
Financial instruments sold, but not yet purchased, at fair value	114,673	231,909
Level 2 [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,510	5,155
Available-for-sale securities	6,923	9,758
Level 2 [Member] | Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	105,447	109,537
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9,786	9,326
Investments	48,865	16,340
Total Assets	58,651	25,666
Financial instruments sold, but not yet purchased, at fair value		955
Level 3 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	175	175
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,060	1,060
Level 3 [Member] | Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9,611	9,151
Financial instruments sold, but not yet purchased, at fair value		955
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	12,621	14,243
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	183	184
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 35,001	$ 853