Investment and Mortgage-Backed Securities, Available for Sale (Narrative) (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 376,910,106		$ 380,374,568
Available-for-sale Securities, Fair Value Disclosure	373,752,293		384,973,906
Available For Sale Securities Pledged as Collateral, Amortized Cost Basis	118,900,000		$ 99,200,000
Proceeds From Sale of Available For Sale Securities Including Mortgage Backed Securities	$ 28,600,000	$ 46,800,000
Percent of unrealized losses for securities in a continuous loss position for 12 months or more	47.10%		34.60%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	64		30
Available-for-sale Securities, Gross Realized Losses	$ 67,000	162,000
GNMA Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	82,100,000		$ 101,300,000
Available-for-sale Securities, Fair Value Disclosure	81,500,000		102,100,000
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	29,900,000		26,900,000
Available-for-sale Securities, Fair Value Disclosure	29,800,000
Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 503,000	$ 870,000	$ 1,288