Investment in Securities, Summary of Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	$ 17,540,000	$ 18,838,000
Gross unrealized gains	19,205,000	18,393,000
Gross unrealized losses	0	0
Fair value	36,745,000	37,231,000
Changes in net unrealized gain (loss), net of tax	500,000	2,300,000	$ (5,500,000)
Amount reclassified from other comprehensive income	2,000,000	800,000	2,800,000
Proceeds from sale of investment available for sale	4,169,000	408,000	81,000
Gross realized gains on sale of investments available for sale	62,000	4,000	6,000
Gross realized losses on sale of investments available for sale	0	0	$ 0
Common Stock [Member]
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	17,441,000	18,739,000
Gross unrealized gains	19,196,000	18,392,000
Gross unrealized losses	0	0
Fair value	36,637,000	37,131,000
Closed-end Funds [Member]
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	99,000	99,000
Gross unrealized gains	9,000	1,000
Gross unrealized losses	0	0
Fair value	$ 108,000	$ 100,000