Loans Receivable - Credit Quality Indicators (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Pass	$ 3,518,105	$ 2,730,283
OAEM	53,640	43,942
Substandard	78,382	70,962
Doubtful/Loss	524	525
Total	3,650,651	2,845,712
Potential problem loans receivable	101,300	83,500
Commercial business
Financing Receivable, Recorded Investment [Line Items]
Pass	2,812,699	2,148,602
OAEM	53,330	42,698
Substandard	71,854	62,840
Doubtful/Loss	0	0
Total	2,937,883	2,254,140
Commercial business | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Pass	788,395	597,697
OAEM	16,168	19,536
Substandard	49,043	28,163
Doubtful/Loss	0	0
Total	853,606	645,396
Commercial business | Owner-occupied commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Pass	741,227	595,455
OAEM	27,724	12,668
Substandard	10,863	14,027
Doubtful/Loss	0	0
Total	779,814	622,150
Commercial business | Non-owner occupied commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Pass	1,283,077	955,450
OAEM	9,438	10,494
Substandard	11,948	20,650
Doubtful/Loss	0	0
Total	1,304,463	986,594
One-to-four family residential
Financing Receivable, Recorded Investment [Line Items]
Pass	100,401	85,762
OAEM	0	0
Substandard	1,362	1,235
Doubtful/Loss	0	0
Total	101,763	86,997
Real estate construction and land development
Financing Receivable, Recorded Investment [Line Items]
Pass	214,197	146,329
OAEM	310	1,244
Substandard	953	1,911
Doubtful/Loss	0	0
Total	215,460	149,484
Real estate construction and land development | One-to-four family residential
Financing Receivable, Recorded Investment [Line Items]
Pass	101,519	49,925
OAEM	258	537
Substandard	953	1,523
Doubtful/Loss	0	0
Total	102,730	51,985
Real estate construction and land development | Five or more family residential and commercial properties
Financing Receivable, Recorded Investment [Line Items]
Pass	112,678	96,404
OAEM	52	707
Substandard	0	388
Doubtful/Loss	0	0
Total	112,730	97,499
Consumer
Financing Receivable, Recorded Investment [Line Items]
Pass	390,808	349,590
OAEM	0	0
Substandard	4,213	4,976
Doubtful/Loss	524	525
Total	$ 395,545	$ 355,091