Loans Receivable - Past Due Loans (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 16,934,000	$ 15,445,000
Current	3,633,717,000	2,830,267,000
Total	3,650,651,000	2,845,712,000
90 Days or More and Still Accruing	0	0
30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,358,000	7,716,000
90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,576,000	7,729,000
Commercial business
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13,240,000	10,851,000
Current	2,924,643,000	2,243,289,000
Total	2,937,883,000	2,254,140,000
Commercial business | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,898,000	5,140,000
Commercial business | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,342,000	5,711,000
Commercial business | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,269,000	4,165,000
Current	848,337,000	641,231,000
Total	853,606,000	645,396,000
Commercial business | Commercial and industrial | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,988,000	2,993,000
Commercial business | Commercial and industrial | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,281,000	1,172,000
Commercial business | Owner-occupied commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,163,000	2,502,000
Current	778,651,000	619,648,000
Total	779,814,000	622,150,000
Commercial business | Owner-occupied commercial real estate | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	563,000	1,277,000
Commercial business | Owner-occupied commercial real estate | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	600,000	1,225,000
Commercial business | Non-owner occupied commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,808,000	4,184,000
Current	1,297,655,000	982,410,000
Total	1,304,463,000	986,594,000
Commercial business | Non-owner occupied commercial real estate | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,347,000	870,000
Commercial business | Non-owner occupied commercial real estate | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,461,000	3,314,000
One-to-four family residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	227,000	513,000
Current	101,536,000	86,484,000
Total	101,763,000	86,997,000
One-to-four family residential | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	227,000	513,000
One-to-four family residential | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Real estate construction and land development
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	899,000	1,455,000
Current	214,561,000	148,029,000
Total	215,460,000	149,484,000
Real estate construction and land development | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	665,000	124,000
Real estate construction and land development | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	234,000	1,331,000
Real estate construction and land development | One-to-four family residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	899,000	1,415,000
Current	101,831,000	50,570,000
Total	102,730,000	51,985,000
Real estate construction and land development | One-to-four family residential | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	665,000	84,000
Real estate construction and land development | One-to-four family residential | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	234,000	1,331,000
Real estate construction and land development | Five or more family residential and commercial properties
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	40,000
Current	112,730,000	97,459,000
Total	112,730,000	97,499,000
Real estate construction and land development | Five or more family residential and commercial properties | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	40,000
Real estate construction and land development | Five or more family residential and commercial properties | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,568,000	2,626,000
Current	392,977,000	352,465,000
Total	395,545,000	355,091,000
Consumer | 30-89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,568,000	1,939,000
Consumer | 90 Days or Greater
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0	$ 687,000