Income Taxes (Narrative) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Income tax expense	$ 5,339,000	$ 5,127,000	$ 4,414,000	$ 3,852,000	$ 3,336,000	$ 3,656,000	$ 3,479,000	$ 3,654,000	$ 18,732,000	$ 14,125,000	$ 3,875,000
Federal Statutory Tax Rate Under Tax Cuts and Jobs Act	21.00%								21.00%
Deferred Tax Assets, Operating Loss Carryforwards	$ 348,000								$ 348,000
Deferred Tax Assets, Gross	11,792,000				20,827,000				11,792,000	20,827,000
Valuation allowance	805,000				$ 1,341,000				805,000	1,341,000
Federal Income Tax Expense on Securities Transactions									1,000,000	$ 326,000	$ 255,000
Partnership investment
Valuation Allowance [Line Items]
Deferred Tax Assets, Gross	$ 3,800,000								3,800,000
Adjustments for New Accounting Pronouncement [Member]
Valuation Allowance [Line Items]
Income tax expense									$ 900,000