Investment and Mortgage-Backed Securities, Available for Sale (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 358,328,622		$ 380,374,568
Available-for-sale Securities, Fair Value Disclosure	358,852,200		384,973,906
Available For Sale Securities Pledged as Collateral, Amortized Cost Basis	117,700,000		99,200,000
Available-for-sale Securities Pledged as Collateral, Fair Value	118,000,000		$ 100,500,000
Proceeds From Sale of Available For Sale Securities Including Mortgage Backed Securities	$ 28,600,000	$ 15,300,000
Percent of unrealized losses for securities in a continuous loss position for 12 months or more	25.40%		34.60%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	31		30
Available-for-sale Securities, Gross Realized Losses	$ 67,000
GNMA Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	83,300,000		$ 101,300,000
Available-for-sale Securities, Fair Value Disclosure	83,200,000		102,100,000
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	30,300,000		26,900,000
Available-for-sale Securities, Fair Value Disclosure			$ 26,900,000
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 503,000	$ 583,000