Fair Value Measurements - Fair Value on Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Securities available-for-sale, at fair value	$ 987,076	$ 710,570
Securities held-to-maturity	251,417	236,125
Loans held-for-sale	1,860	0
Derivative assets		490
Financial liabilities:
Derivative liabilities		203
Quoted prices in active markets for identical assets (Level 1)
Financial assets:
Cash and cash equivalents	54,834	65,498
Securities available-for-sale, at fair value		42,831
Securities held-to-maturity	0	0
Federal Home Loan Bank stock	0	0
Loans held-for-sale	0
Loans receivable, net	0	0
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities	0	0
Significant other observable inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale, at fair value		667,739
Securities held-to-maturity		236,125
Federal Home Loan Bank stock	56,532	44,664
Loans held-for-sale	1,860
Loans receivable, net	0	0
Derivative assets	2,625	490
Financial liabilities:
Total deposits	4,355,309	3,959,460
Borrowings	1,127,940	925,032
Derivative liabilities	230	203
Significant unobservable inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale, at fair value	0	0
Securities held-to-maturity	0	0
Federal Home Loan Bank stock	0	0
Loans held-for-sale	0
Loans receivable, net	4,706,875	4,367,945
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities	0	0
Carrying Value
Financial assets:
Cash and cash equivalents	54,834	65,498
Securities available-for-sale, at fair value	987,076	710,570
Securities held-to-maturity	264,184	239,618
Federal Home Loan Bank stock	56,532	44,664
Loans held-for-sale	1,860
Loans receivable, net	4,843,297	4,400,470
Derivative assets	2,625	490
Financial liabilities:
Total deposits	4,372,345	4,263,315
Borrowings	1,135,730	929,057
Derivative liabilities	230	203
Total Fair Value
Financial assets:
Cash and cash equivalents	54,834	65,498
Securities available-for-sale, at fair value	987,076	710,570
Securities held-to-maturity	251,417	236,125
Federal Home Loan Bank stock	56,532	44,664
Loans held-for-sale	1,860
Loans receivable, net	4,706,875	4,367,945
Derivative assets	2,625	490
Financial liabilities:
Total deposits	4,355,309	3,959,460
Borrowings	1,127,940	925,032
Derivative liabilities	$ 230	$ 203