Investment and Mortgage-Backed Securities, Available for Sale (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 398,638,123		$ 386,296,513
Available-for-sale Securities, Fair Value Disclosure	402,351,692		386,255,837
Available For Sale Securities Pledged as Collateral, Amortized Cost Basis	120,600,000		$ 111,800,000
Proceeds From Sale of Available For Sale Securities Including Mortgage Backed Securities	$ 6,600,000	$ 28,600,000
Percent of unrealized losses for securities in a continuous loss position for 12 months or more	47.10%		72.40%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	86		92
Available-for-sale Securities, Gross Realized Losses	$ 8,000	67,000
GNMA Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	78,200,000		$ 80,400,000
Available-for-sale Securities, Fair Value Disclosure	78,700,000		80,200,000
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	29,300,000		29,700,000
Available-for-sale Securities, Fair Value Disclosure	29,400,000
Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 299,000	$ 503,000	$ 20,713