Fair Value Measurements - Fair Value on Balance Sheet (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Debt securities available for sale, at fair value	$ 1,090,177,000	$ 1,032,868,000
Debt securities held to maturity	284,450,000	254,841,000
Equity securities	1,428,000	1,890,000
Loans held-for-sale	0	8,100,000
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	65,141,000	42,201,000
Debt securities available for sale, at fair value	54,517,000	54,157,000
Debt securities held to maturity	33,426,000	23,241,000
Equity securities	1,428,000	1,890,000
Federal Home Loan Bank stock	0	0
Loans held-for-sale		0
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, at fair value	1,035,660,000	978,711,000
Debt securities held to maturity	251,024,000	231,600,000
Equity securities	0	0
Federal Home Loan Bank stock	54,863,000	58,938,000
Loans held-for-sale		8,081,000
Loans receivable, net	0	0
Derivative assets	2,213,000	1,342,000
Financial liabilities:
Deposits	4,601,963,000	4,402,336,000
Borrowings	1,097,897,000	1,185,007,000
Derivative liabilities	8,271,000	3,944,000
Significant Unobservable Inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Debt securities available for sale, at fair value	0	0
Debt securities held to maturity	0	0
Equity securities	0	0
Federal Home Loan Bank stock	0	0
Loans held-for-sale		0
Loans receivable, net	4,917,066,000	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	65,141,000	42,201,000
Debt securities available for sale, at fair value	1,090,177,000	1,032,868,000
Debt securities held to maturity	287,529,000	262,143,000
Equity securities	1,428,000	1,890,000
Federal Home Loan Bank stock	54,863,000	58,938,000
Loans held-for-sale		8,081,000
Loans receivable, net	4,948,578,000	4,916,840,000
Derivative assets	2,213,000	1,342,000
Financial liabilities:
Deposits	4,606,628,000	4,413,873,000
Borrowings	1,098,635,000	1,189,180,000
Derivative liabilities	8,271,000	3,944,000
Total Fair Value
Financial assets:
Cash and cash equivalents	65,141,000	42,201,000
Debt securities available for sale, at fair value	1,090,177,000	1,032,868,000
Debt securities held to maturity	284,450,000	254,841,000
Equity securities	1,428,000	1,890,000
Federal Home Loan Bank stock	54,863,000	58,938,000
Loans held-for-sale		8,081,000
Loans receivable, net	4,917,066,000	4,841,830,000
Derivative assets	2,213,000	1,342,000
Financial liabilities:
Deposits	4,601,963,000	4,402,336,000
Borrowings	1,097,897,000	1,185,007,000
Derivative liabilities	$ 8,271,000	$ 3,944,000