Investment and Mortgage-Backed Securities, Available for Sale (Narrative) (Details)	12 Months Ended
Dec. 31, 2020 USD ($) security	Mar. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 572,137,595	$ 558,836,046
Available-for-sale Securities, Fair Value Disclosure	589,324,401	568,787,583
Available For Sale Securities Pledged as Collateral, Amortized Cost Basis	$ 263,300,000	$ 288,000,000.0
Percent of unrealized losses for securities in a continuous loss position for 12 months or more	37.00%	21.80%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| security	88	84
GNMA Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost of Investment And Mortgage-Backed Securities Available For Sale	$ 91,700,000	$ 88,900,000
Available-for-sale Securities, Fair Value Disclosure	93,400,000	90,600,000
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	37,000,000.0	32,900,000
Available-for-sale Securities, Fair Value Disclosure		$ 33,400,000
Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 220,239

Available For Sale, Percent of Unrealized Losses for Securities in a Continuous Loss Position, 12 Months or Longer

No definition available.

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- Definition

Available For Sale Securities Pledged as Collateral, Amortized Cost Basis

+ References

No definition available.

+ Details

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

+ Details

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

+ Details

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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