Investment in Securities, Part I (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Available for sale securities:
Available-for-sale Securities, Cost	$ 45,395,000	$ 55,759,000	$ 45,395,000	$ 55,759,000	$ 53,850,000
Available for sale securities, Fair Value	87,292,000	94,370,000	87,292,000	94,370,000	97,114,000
Investments in securities	220,440,000	249,630,000	220,440,000	249,630,000	218,843,000
Amount Reclassified from AOCI [Abstract]
Net gain from investments - Realized gain / (loss) on sale of securities	9,849,000		10,446,000
Other operating expenses	181,000		331,000
Other Comprehensive Income (Loss), Reclassification Adjustment for Write-down of Securities Included in Net Income, before Tax	(14,000)		(14,000)
Income before income taxes	10,016,000		10,763,000
Income tax provision	(3,706,000)		(3,982,000)
Net income	6,310,000		6,781,000
Gross Amounts Not Offset in the Statements of Financial Position, Liabilities [Abstract]
Net gain/(loss) on equity derivative	(1,000)	15,000	280,000	(14,000)
Shares under derivative contract (in shares)	1.5	4.7	1.5	4.7	1.2
Derivative fair value of derivative net	122,000	(72,000)	122,000	(72,000)	(121,000)
Swaps [Member]
Offsetting Derivative Assets [Abstract]
Gross amounts of recognized assets	237,000	942,000	237,000	942,000	148,000
Gross amounts offset in the Statements of Financial Position	0	0	0	0	0
Net Amounts of Assets Presented in the Statements of Financial Position	237,000	942,000	237,000	942,000	148,000
Gross Amounts Not Offset in the Statements of Financial Position, Assets [Abstract]
Financial Instruments	(177,000)	(942,000)	(177,000)	(942,000)	(132,000)
Cash collateral received	0	0	0	0	0
Net amount	60,000	0	60,000	0	16,000
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized liabilities	177,000	1,014,000	177,000	1,014,000	132,000
Gross amounts offset in the Statements of Financial Position	0	0	0	0	0
Net Amounts of Assets Presented in the Statements of Financial Position	177,000	1,014,000	177,000	1,014,000	132,000
Gross Amounts Not Offset in the Statements of Financial Position, Liabilities [Abstract]
Financial Instruments	(177,000)	(942,000)	(177,000)	(942,000)	(132,000)
Cash collateral received	0	0	0	0	0
Net amount	0	72,000	0	72,000	0
Investment in Securities [Member]
Trading securities:
Trading Securities, Cost	182,050,000	223,293,000	182,050,000	223,293,000	185,997,000
Trading securities, fair value	220,440,000	249,630,000	220,440,000	249,630,000	218,843,000
Investment in Securities [Member] | Trading Securities [Member]
Trading securities:
Trading Securities, Cost	167,130,000	205,998,000	167,130,000	205,998,000	170,070,000
Trading securities, fair value	185,478,000	214,802,000	185,478,000	214,802,000	183,581,000
Trading Securities:
Cost	7,739,000	6,206,000	7,739,000	6,206,000	2,777,000
Fair Value	7,598,000	7,010,000	7,598,000	7,010,000	3,136,000
Investment in Securities [Member] | Trading Securities [Member] | Government obligations [Member]
Trading securities:
Trading Securities, Cost	27,986,000	50,119,000	27,986,000	50,119,000	42,973,000
Trading securities, fair value	27,996,000	50,139,000	27,996,000	50,139,000	42,989,000
Investment in Securities [Member] | Trading Securities [Member] | Common Stock [Member]
Trading securities:
Trading Securities, Cost	137,600,000	154,470,000	137,600,000	154,470,000	125,697,000
Trading securities, fair value	155,088,000	162,883,000	155,088,000	162,883,000	138,478,000
Trading Securities:
Cost	7,668,000	6,194,000	7,668,000	6,194,000	2,593,000
Fair Value	7,381,000	6,687,000	7,381,000	6,687,000	2,867,000
Investment in Securities [Member] | Trading Securities [Member] | Mutual Funds [Member]
Trading securities:
Trading Securities, Cost	1,074,000	1,086,000	1,074,000	1,086,000	1,072,000
Trading securities, fair value	1,675,000	1,406,000	1,675,000	1,406,000	1,484,000
Investment in Securities [Member] | Trading Securities [Member] | Other Investments [Member]
Trading securities:
Trading Securities, Cost	470,000	323,000	470,000	323,000	328,000
Trading securities, fair value	719,000	374,000	719,000	374,000	630,000
Trading Securities:
Cost	71,000	12,000	71,000	12,000	184,000
Fair Value	217,000	323,000	217,000	323,000	269,000
Investment in Securities [Member] | Available-for-sale Securities [Member]
Trading securities:
Trading Securities, Cost	14,920,000	17,295,000	14,920,000	17,295,000	15,927,000
Trading securities, fair value	34,962,000	34,828,000	34,962,000	34,828,000	35,262,000
Investment in Securities [Member] | Available-for-sale Securities [Member] | Common Stock [Member]
Trading securities:
Trading Securities, Cost	14,077,000	15,934,000	14,077,000	15,934,000	14,822,000
Trading securities, fair value	33,478,000	32,815,000	33,478,000	32,815,000	33,560,000
Investment in Securities [Member] | Available-for-sale Securities [Member] | Mutual Funds [Member]
Trading securities:
Trading Securities, Cost	843,000	1,361,000	843,000	1,361,000	1,105,000
Trading securities, fair value	1,484,000	2,013,000	1,484,000	2,013,000	1,702,000
Investment in Sponsored Registered Investment Companies [Member]
Available for sale securities:
Available-for-sale Securities, Cost	30,494,000	38,483,000	30,494,000	38,483,000	37,942,000
Available for sale securities, Fair Value	52,341,000	59,561,000	52,341,000	59,561,000	61,872,000
Investment in Sponsored Registered Investment Companies [Member] | Trading Securities [Member]
Available for sale securities:
Available-for-sale Securities, Cost	19,000	19,000	19,000	19,000	19,000
Available for sale securities, Fair Value	11,000	19,000	11,000	19,000	20,000
Investment in Sponsored Registered Investment Companies [Member] | Trading Securities [Member] | Mutual Funds [Member]
Available for sale securities:
Available-for-sale Securities, Cost	19,000	19,000	19,000	19,000	19,000
Available for sale securities, Fair Value	11,000	19,000	11,000	19,000	20,000
Investment in Sponsored Registered Investment Companies [Member] | Available-for-sale Securities [Member]
Available for sale securities:
Available-for-sale Securities, Cost	30,475,000	38,464,000	30,475,000	38,464,000	37,923,000
Available for sale securities, Fair Value	52,330,000	59,542,000	52,330,000	59,542,000	61,852,000
Investment in Sponsored Registered Investment Companies [Member] | Available-for-sale Securities [Member] | Mutual Funds [Member]
Available for sale securities:
Available-for-sale Securities, Cost	2,040,000	2,198,000	2,040,000	2,198,000	2,055,000
Available for sale securities, Fair Value	3,168,000	3,371,000	3,168,000	3,371,000	3,341,000
Investment in Sponsored Registered Investment Companies [Member] | Available-for-sale Securities [Member] | Closed end Funds [Member]
Available for sale securities:
Available-for-sale Securities, Cost	28,435,000	36,266,000	28,435,000	36,266,000	35,868,000
Available for sale securities, Fair Value	$ 49,162,000	$ 56,171,000	$ 49,162,000	$ 56,171,000	$ 58,511,000