Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,352,359	$ 1,267,449
Available-for-sale securities	2,703,060	3,070,447
Investments	81,841	67,982
Cash equivalents measured at NAV	31,590	19,719
Derivative contracts, Assets	2,187	7,683
Total Assets	4,171,037	4,433,280
Financial instruments sold, but not yet purchased, at fair value	956,344	947,306
Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	800	14,063
Financial instruments sold, but not yet purchased, at fair value	1,542	11,429
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	130,141	72,532
Available-for-sale securities	300	5,215
Financial instruments sold, but not yet purchased, at fair value	4,011	32,755
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,220	25,905
Available-for-sale securities	1,582,388	1,764,060
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	326,407	310,457
Available-for-sale securities	797,521	931,604
Financial instruments sold, but not yet purchased, at fair value	248,962	208,725
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	82,968	57,911
Investments	40,318	33,046
Financial instruments sold, but not yet purchased, at fair value	66,312	36,117
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	193,859	154,622
Available-for-sale securities	43,301	68,226
Financial instruments sold, but not yet purchased, at fair value		7
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,732	16,632
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	654	704
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	10,326	1,041
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	13,811	16,559
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	56,478	42,121
Financial instruments sold, but not yet purchased, at fair value	389,489	534,817
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	545,340	564,111
Available-for-sale securities	200,495	230,408
Financial instruments sold, but not yet purchased, at fair value	246,028	123,456
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	11,146	25,727
Available-for-sale securities	1,090	1,219
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	77,965	69,715
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	141,967	100,346
Available-for-sale securities	300	417
Investments	48,457	31,670
Total Assets	190,724	132,433
Financial instruments sold, but not yet purchased, at fair value	456,033	571,504
Level 1 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	300	417
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,552	1,100
Financial instruments sold, but not yet purchased, at fair value	232	1,289
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	82,937	57,125
Investments	39,258	31,670
Financial instruments sold, but not yet purchased, at fair value	66,312	35,398
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	9,199
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	56,478	42,121
Financial instruments sold, but not yet purchased, at fair value	389,489	534,817
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,210,217	1,166,927
Available-for-sale securities	2,702,760	3,070,030
Investments	1,804	1,560
Derivative contracts, Assets	2,187	7,683
Total Assets	3,916,968	4,246,200
Financial instruments sold, but not yet purchased, at fair value	500,311	375,802
Level 2 [Member] | Sovereign Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	800	14,063
Financial instruments sold, but not yet purchased, at fair value	1,542	11,429
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	130,141	72,532
Available-for-sale securities		4,798
Financial instruments sold, but not yet purchased, at fair value	4,011	32,755
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,045	25,730
Available-for-sale securities	1,582,388	1,764,060
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	323,855	309,357
Available-for-sale securities	797,521	931,604
Financial instruments sold, but not yet purchased, at fair value	248,730	207,436
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	31	786
Investments	1,060	1,376
Financial instruments sold, but not yet purchased, at fair value		719
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	193,859	154,622
Available-for-sale securities	43,301	68,226
Financial instruments sold, but not yet purchased, at fair value		7
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	274	184
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	545,340	564,111
Available-for-sale securities	200,495	230,408
Financial instruments sold, but not yet purchased, at fair value	246,028	123,456
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	11,146	25,726
Available-for-sale securities	1,090	1,219
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	77,965	69,715
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	175	176
Investments	17,769	18,193
Total Assets	17,944	18,369
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] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,732	16,632
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	184	704
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 853	857
Level 3 [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