Fair Value Measurements - Fair Value on Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Securities available-for-sale, at fair value	$ 932,070	$ 710,570
Securities held-to-maturity	244,239	236,125
Quoted prices in active markets for identical assets (Level 1)
Financial assets:
Cash and cash equivalents	67,183	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 receivable, net	0	0
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities		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	45,009	44,664
Loans receivable, net	0	0
Derivative assets	965	490
Financial liabilities:
Total deposits	4,282,179	3,959,460
Borrowings	923,606	925,032
Derivative liabilities		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 receivable, net	4,538,326	4,367,945
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities		0
Carrying Value
Financial assets:
Cash and cash equivalents	67,183	65,498
Securities available-for-sale, at fair value	932,070	710,570
Securities held-to-maturity	254,801	239,618
Federal Home Loan Bank stock	45,009	44,664
Loans receivable, net	4,649,054	4,400,470
Derivative assets	965	490
Financial liabilities:
Total deposits	4,294,832	4,263,315
Borrowings	930,618	929,057
Derivative liabilities		203
Total Fair Value
Financial assets:
Cash and cash equivalents	67,183	65,498
Securities available-for-sale, at fair value	932,070	710,570
Securities held-to-maturity	244,239	236,125
Federal Home Loan Bank stock	45,009	44,664
Loans receivable, net	4,538,326	4,367,945
Derivative assets	965	490
Financial liabilities:
Total deposits	4,282,179	3,959,460
Borrowings	$ 923,606	925,032
Derivative liabilities		$ 203