Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Sep. 30, 2014
Credit Risk Grades
Totals	$ 2,857,775	$ 2,262,683	$ 2,130,853
Commercial and industrial
Credit Risk Grades
Totals	491,112	309,741	276,321	$ 276,321
Franchise
Credit Risk Grades
Totals	371,875	328,925	216,544
Commercial owner occupied
Credit Risk Grades
Totals	424,289	294,726	279,704
SBA
Credit Risk Grades
Totals	78,350	62,256	49,855
Warehouse facilities
Credit Risk Grades
Totals	1,394	143,200	216,554
Commercial non-owner occupied
Credit Risk Grades
Totals	522,080	421,583	452,422
Multi-family
Credit Risk Grades
Totals	619,485	429,003	397,130
One-to-four family
Credit Risk Grades
Totals	106,854	80,050	116,735
Construction
Credit Risk Grades
Totals	218,069	169,748	111,704	111,704
Land
Credit Risk Grades
Totals	18,222	18,340	7,243
Other loans
Credit Risk Grades
Totals	6,045	5,111	6,641	$ 6,641
Pass
Credit Risk Grades
Totals	2,828,668	2,242,242	2,097,145
Pass | Commercial and industrial
Credit Risk Grades
Totals	481,770	306,513	268,285
Pass | Franchise
Credit Risk Grades
Totals	370,245	327,295	214,690
Pass | Commercial owner occupied
Credit Risk Grades
Totals	413,627	286,270	268,038
Pass | SBA
Credit Risk Grades
Totals	78,280	62,256	49,855
Pass | Warehouse facilities
Credit Risk Grades
Totals	1,394	143,200	216,554
Pass | Commercial non-owner occupied
Credit Risk Grades
Totals	520,248	418,917	446,900
Pass | Multi-family
Credit Risk Grades
Totals	616,620	425,616	391,690
Pass | One-to-four family
Credit Risk Grades
Totals	104,608	78,997	115,780
Pass | Construction
Credit Risk Grades
Totals	218,069	169,748	111,469
Pass | Land
Credit Risk Grades
Totals	18,203	18,319	7,243
Pass | Other loans
Credit Risk Grades
Totals	5,604	5,111	6,641
Special Mention
Credit Risk Grades
Totals	5,006	700	3,204
Special Mention | Commercial and industrial
Credit Risk Grades
Totals	2,977	73	1,250
Special Mention | Franchise
Credit Risk Grades
Totals	0	0	0
Special Mention | Commercial owner occupied
Credit Risk Grades
Totals	2,029	627	0
Special Mention | SBA
Credit Risk Grades
Totals	0	0	0
Special Mention | Warehouse facilities
Credit Risk Grades
Totals	0	0	0
Special Mention | Commercial non-owner occupied
Credit Risk Grades
Totals	0	0	0
Special Mention | Multi-family
Credit Risk Grades
Totals	0	0	1,954
Special Mention | One-to-four family
Credit Risk Grades
Totals	0	0	0
Special Mention | Construction
Credit Risk Grades
Totals	0	0	0
Special Mention | Land
Credit Risk Grades
Totals	0	0	0
Special Mention | Other loans
Credit Risk Grades
Totals	0	0	0
Substandard
Credit Risk Grades
Totals	22,640	18,280	30,504
Substandard | Commercial and industrial
Credit Risk Grades
Totals	6,365	3,155	6,786
Substandard | Franchise
Credit Risk Grades
Totals	169	169	1,854
Substandard | Commercial owner occupied
Credit Risk Grades
Totals	8,633	7,829	11,666
Substandard | SBA
Credit Risk Grades
Totals	70	0	0
Substandard | Warehouse facilities
Credit Risk Grades
Totals	0	0	0
Substandard | Commercial non-owner occupied
Credit Risk Grades
Totals	1,832	2,666	5,522
Substandard | Multi-family
Credit Risk Grades
Totals	2,865	3,387	3,486
Substandard | One-to-four family
Credit Risk Grades
Totals	2,246	1,053	955
Substandard | Construction
Credit Risk Grades
Totals	0	0	235
Substandard | Land
Credit Risk Grades
Totals	19	21	0
Substandard | Other loans
Credit Risk Grades
Totals	441	0	0
Doubtful
Credit Risk Grades
Totals	1,461	1,461	0
Doubtful | Commercial and industrial
Credit Risk Grades
Totals	0	0	0
Doubtful | Franchise
Credit Risk Grades
Totals	1,461	1,461	0
Doubtful | Commercial owner occupied
Credit Risk Grades
Totals	0	0	0
Doubtful | SBA
Credit Risk Grades
Totals	0	0	0
Doubtful | Warehouse facilities
Credit Risk Grades
Totals	0	0	0
Doubtful | Commercial non-owner occupied
Credit Risk Grades
Totals	0	0	0
Doubtful | Multi-family
Credit Risk Grades
Totals	0	0	0
Doubtful | One-to-four family
Credit Risk Grades
Totals	0	0	0
Doubtful | Construction
Credit Risk Grades
Totals	0	0	0
Doubtful | Land
Credit Risk Grades
Totals	0	0	0
Doubtful | Other loans
Credit Risk Grades
Totals	$ 0	$ 0	$ 0