CONSOLIDATED STATEMENTS OF INCOME - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
INTEREST INCOME:
Loans, including fees	$ 205,582,000	$ 164,232,000	$ 133,958,000
Taxable investment securities	9,404,000	7,876,000	7,046,000
Federal Home Loan Bank stock and other	3,792,000	1,884,000	1,428,000
Total interest income	218,778,000	173,992,000	142,432,000
INTEREST EXPENSE:
Deposits	9,802,000	6,058,000	4,380,000
Securities sold under agreements to repurchase and other short-term borrowings	502,000	65,000	92,000
Federal Home Loan Bank advances	8,860,000	10,900,000	11,934,000
Subordinated note	1,094,000	915,000	2,056,000
Total interest expense	20,258,000	17,938,000	18,462,000
NET INTEREST INCOME	198,520,000	156,054,000	123,970,000
Provision for loan and lease losses	27,704,000	14,493,000	5,396,000
NET INTEREST INCOME AFTER PROVISION FOR LOAN AND LEASE LOSSES	170,816,000	141,561,000	118,574,000
NONINTEREST INCOME:
Service charges on deposit accounts	13,357,000	13,176,000	13,015,000
Net refund transfer fees	18,500,000	19,240,000	17,388,000
Mortgage banking income	4,642,000	6,882,000	4,411,000
Interchange fee income	9,881,000	9,009,000	8,353,000
Program fees	5,824,000	3,044,000	1,233,000
Increase in cash surrender value of bank owned life insurance	1,562,000	1,516,000	1,402,000
Net gains (losses) on securities available for sale	(136,000)		88,000
Net gains (losses) on other real estate owned	676,000	244,000	(301,000)
Other	4,108,000	4,398,000	2,405,000
Total noninterest income	58,414,000	57,509,000	47,994,000
NONINTEREST EXPENSE:
Salaries and employee benefits	82,233,000	69,882,000	58,091,000
Occupancy and equipment, net	24,019,000	21,586,000	20,557,000
Communication and transportation	4,711,000	4,256,000	3,752,000
Marketing and development	5,188,000	3,778,000	3,161,000
FDIC insurance expense	1,378,000	1,780,000	2,084,000
Bank franchise tax expense	4,626,000	4,757,000	4,734,000
Data processing	7,748,000	6,121,000	4,340,000
Interchange related expense	3,988,000	4,140,000	3,873,000
Supplies	1,594,000	1,406,000	1,101,000
Other real estate owned expense	388,000	503,000	735,000
Legal and professional fees	2,410,000	2,556,000	3,306,000
FHLB advance prepayment penalty	0	846,000	0
Impairment of premises held for sale	1,175,000	191,000	132,000
Other	11,386,000	8,305,000	7,458,000
Total noninterest expense	150,844,000	130,107,000	113,324,000
INCOME BEFORE INCOME TAX EXPENSE	78,386,000	68,963,000	53,244,000
INCOME TAX EXPENSE:
Income tax expense - TCJA	6,326,000
Income tax expense - Other	26,428,000	23,060,000	18,078,000
Total income tax expense	32,754,000	23,060,000	18,078,000
NET INCOME	$ 45,632,000	$ 45,903,000	$ 35,166,000
Class A Common Stock
BASIC EARNINGS PER SHARE:
Basic earnings per share (in dollars per share)	$ 2.21	$ 2.22	$ 1.70
DILUTED EARNINGS PER SHARE:
Diluted earnings per share (in dollars per share)	2.20	2.22	1.70
DIVIDENDS DECLARED PER COMMON SHARE:
Dividend declared per common share (in dollars per share)	0.869	0.825	0.781
Class B Common Stock
BASIC EARNINGS PER SHARE:
Basic earnings per share (in dollars per share)	2.01	2.02	1.55
DILUTED EARNINGS PER SHARE:
Diluted earnings per share (in dollars per share)	2.00	2.01	1.54
DIVIDENDS DECLARED PER COMMON SHARE:
Dividend declared per common share (in dollars per share)	$ 0.790	$ 0.750	$ 0.710