Loans - Schedule of Loans by Class (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable
Loans held for investment	$ 7,567,720	$ 5,416,750
Mortgage loans held for sale	46,635	61,794
Total loans	7,614,355	5,478,544
Commercial
Loans and Leases Receivable
Loans held for investment	$ 2,822,900	1,834,400
Loans receivable, maturity	5 years
Land acquisition & development
Loans and Leases Receivable
Loans held for investment	$ 348,700	208,500
Residential
Loans and Leases Receivable
Loans held for investment	240,200	147,900
Commercial
Loans and Leases Receivable
Loans held for investment	119,400	125,600
Total construction loans
Loans and Leases Receivable
Loans held for investment	708,300	482,000
Residential
Loans and Leases Receivable
Loans held for investment	$ 1,487,400	1,027,400
Loans receivable, maturity	15 years
Agricultural
Loans and Leases Receivable
Loans held for investment	$ 158,200	170,200
Total real estate loans
Loans and Leases Receivable
Loans held for investment	5,176,800	3,514,000
Indirect consumer
Loans and Leases Receivable
Loans held for investment	784,700	752,400
Other consumer
Loans and Leases Receivable
Loans held for investment	175,100	148,100
Credit card consumer
Loans and Leases Receivable
Loans held for investment	74,600	69,800
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,034,400	970,300
Commercial
Loans and Leases Receivable
Loans held for investment	1,215,400	797,900
Agricultural
Loans and Leases Receivable
Loans held for investment	136,200	132,900
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	4,900	1,600
Loans held for investment
Loans and Leases Receivable
Loans held for investment	7,567,700	5,416,700
Total loans
Loans and Leases Receivable
Total loans	7,614,300	5,478,500
Home equity line of credit
Loans and Leases Receivable
Loans held for investment	$ 397,000	$ 321,500
Loans receivable, loan to value limit	80.00%