Loans Receivable and Allowance for Credit Losses - Past Due Financing Receivables (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	$ 3,028,000	$ 5,376,000
Accruing current balances	459,461,000	360,127,000
Nonaccrual loans	4,071,000	6,980,000
Total loans receivables	466,560,000	372,483,000	$ 333,274,000
Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	1,578,000	3,398,000
Nonaccrual loans	725,000	964,000
Financing Receivables, 1 to 29 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	1,029,000	464,000
Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	1,076,000	1,978,000
Nonaccrual loans	0	30,000
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	374,000	0
Nonaccrual loans	2,317,000	5,521,000
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	9,000	0
Accruing current balances	650,000	446,000
Nonaccrual loans	0	0
Total loans receivables	659,000	446,000	458,000
Consumer | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	9,000	0
Consumer | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Consumer | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Commercial and Industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	46,000	0
Accruing current balances	73,200,000	44,805,000
Nonaccrual loans	70,000	350,000
Total loans receivables	73,316,000	45,155,000	29,297,000
Commercial and Industrial | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	46,000	0
Commercial and Industrial | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Commercial and Industrial | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Accruing current balances	19,067,000	9,934,000
Nonaccrual loans	0	0
Total loans receivables	19,067,000	9,934,000	8,860,000
Construction | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Construction | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Construction | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Commercial and Multi-Family Real Estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	374,000	374,000
Accruing current balances	183,835,000	123,182,000
Nonaccrual loans	329,000	760,000
Total loans receivables	184,538,000	124,316,000	102,804,000
Commercial and Multi-Family Real Estate | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	374,000
Commercial and Multi-Family Real Estate | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0	0
Commercial and Multi-Family Real Estate | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	374,000	0
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans receivables	188,980,000	192,632,000	$ 191,855,000
Residential | One-to-Four Family Occupied
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due		4,377,000
Accruing current balances		150,260,000
Nonaccrual loans		5,744,000
Total loans receivables		160,381,000
Residential | One-to-Four Family Occupied | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due		2,433,000
Residential | One-to-Four Family Occupied | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due		1,944,000
Residential | One-to-Four Family Occupied | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due		0
Residential | One-to-Four Family
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	1,436,000
Accruing current balances	156,583,000
Nonaccrual loans	3,556,000
Total loans receivables	161,575,000
Residential | One-to-Four Family | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	794,000
Residential | One-to-Four Family | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	642,000
Residential | One-to-Four Family | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	0
Residential | Home Equity Loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	1,163,000	625,000
Accruing current balances	26,126,000	31,500,000
Nonaccrual loans	116,000	126,000
Total loans receivables	27,405,000	32,251,000
Residential | Home Equity Loan | Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	729,000	591,000
Residential | Home Equity Loan | Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	434,000	34,000
Residential | Home Equity Loan | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans receivable, past due	$ 0	$ 0