Investment and Mortgage-Backed Securities, Available for Sale (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 363,600,602		$ 363,600,602		$ 380,374,568
Available-for-sale Securities, Fair Value Disclosure	362,949,939		362,949,939		384,973,906
Available For Sale Securities Pledged as Collateral, Amortized Cost Basis	112,800,000		$ 112,800,000		$ 99,200,000
Proceeds From Sale of Available For Sale Securities Including Mortgage Backed Securities	$ 28,600,000			$ 17,500,000
Percent of unrealized losses for securities in a continuous loss position for 12 months or more	28.20%		28.20%		34.60%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	39		39		30
Available-for-sale Securities, Gross Realized Losses	$ 67,000
GNMA Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	78,900,000		$ 78,900,000		$ 101,300,000
Available-for-sale Securities, Fair Value Disclosure	78,700,000		78,700,000		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	30,000,000		30,000,000		$ 26,900,000
Available-for-sale Securities, Fair Value Disclosure	29,900,000		29,900,000
Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 503,000	$ 629,000	$ 7,029	$ 1,288