Fair Value Measurements - Fair Value on Balance Sheet (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Financial assets:
Debt securities available for sale	$ 1,098,336,000	$ 1,032,868,000
Debt securities held to maturity	289,505,000	254,841,000
Equity securities, at fair value	2,855,000	1,890,000
Loans held-for-sale	0	8,100,000
Loans receivable, net	0	8,081,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	75,547,000	42,201,000
Debt securities available for sale		54,157,000
Debt securities held to maturity	19,960,000	23,241,000
Equity securities, at fair value	2,587,000	1,890,000
Federal Home Loan Bank stock	0	0
Loans held-for-sale
Loans receivable, net	0	0
Derivative assets	0	0
Financial liabilities:
Deposits	0	0
Borrowings	0	0
Derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Debt securities available for sale		978,711,000
Debt securities held to maturity	269,545,000	231,600,000
Equity securities, at fair value	268,000	0
Federal Home Loan Bank stock	69,579,000	58,938,000
Loans held-for-sale		8,081,000
Loans receivable, net	0	0
Derivative assets	185,000	1,342,000
Financial liabilities:
Deposits	5,654,075,000	4,402,336,000
Borrowings	1,411,962,000	1,185,007,000
Derivative liabilities	11,546,000	3,944,000
Significant Unobservable Inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Debt securities available for sale	0	0
Debt securities held to maturity	0	0
Equity securities, at fair value		0
Federal Home Loan Bank stock	0	0
Loans held-for-sale		0
Loans receivable, net	6,219,008,000	4,841,830,000
Derivative assets	0	0
Financial liabilities:
Deposits	0	0
Borrowings	0	0
Derivative liabilities	0	0
Carrying Value
Financial assets:
Cash and cash equivalents	75,547,000	42,201,000
Debt securities available for sale	1,098,336,000	1,032,868,000
Debt securities held to maturity	285,756,000	262,143,000
Equity securities, at fair value	2,855,000	1,890,000
Federal Home Loan Bank stock	69,579,000	58,938,000
Loans held-for-sale		8,081,000
Loans receivable, net	6,135,857,000	4,916,840,000
Derivative assets	185,000	1,342,000
Financial liabilities:
Deposits	5,645,842,000	4,413,873,000
Borrowings	1,407,022,000	1,189,180,000
Derivative liabilities	11,546,000	3,944,000
Total Fair Value
Financial assets:
Cash and cash equivalents	75,547,000	42,201,000
Debt securities available for sale	1,098,336,000	1,032,868,000
Debt securities held to maturity	289,505,000	254,841,000
Equity securities, at fair value	2,855,000	1,890,000
Federal Home Loan Bank stock	69,579,000	58,938,000
Loans held-for-sale		8,081,000
Loans receivable, net	6,219,008,000	4,841,830,000
Derivative assets	185,000	1,342,000
Financial liabilities:
Deposits	5,654,075,000	4,402,336,000
Borrowings	1,411,962,000	1,185,007,000
Derivative liabilities	$ 11,546,000	$ 3,944,000