Investment and Mortgage-Backed Securities, Available for Sale (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 382,775,623		$ 382,775,623		$ 360,457,164
Available-for-sale Securities, Fair Value Disclosure	388,643,233		388,643,233		362,059,429
Available For Sale Securities Pledged as Collateral, Amortized Cost Basis	97,600,000		97,600,000		75,300,000
Available-for-sale Securities Pledged as Collateral, Fair Value	99,500,000		99,500,000		$ 76,900,000
Proceeds From Sale of Available For Sale Securities Including Mortgage Backed Securities	$ 29,300,000	$ 6,900,000	$ 46,800,000	$ 21,500,000
Percent of unrealized losses for securities in a continuous loss position for 12 months or more	51.30%		51.30%		3.50%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	28		28		15
Available-for-sale Securities, Gross Realized Losses	$ 162,000,000		$ 162,000
GNMA Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	93,000,000		93,000,000		$ 107,900,000
Available-for-sale Securities, Fair Value Disclosure	94,200,000		94,200,000		109,200,000
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	31,200,000		31,200,000		20,000,000
Available-for-sale Securities, Fair Value Disclosure					$ 19,700,000
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 241,000	$ 360,000	$ 870,000	$ 772,000