Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 76,214	$ 76,800	$ 74,200
Provision charged (credited) to operating expense	11,053	9,991	6,822
Less loans charged-off	(22,900)	(19,800)	(11,700)
Add back recoveries of loans previously charged-off	7,700	9,200	7,500
Ending balance	72,147	76,214	76,800
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				$ 10,800	$ 14,000	$ 11,600
Collectively evaluated for impairment				61,300	62,200	65,200
Ending balance	76,214	76,800	74,200	72,147	76,214	76,800
Total loans:
Individually evaluated for impairment				83,200	104,700	92,000
Collectively evaluated for impairment				7,484,500	5,312,000	5,101,400
Total loans held for investment				7,567,700	5,416,700	5,193,400
Allowance for loans losses				1,000	400
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	28,600	52,300	53,900
Provision charged (credited) to operating expense	6,000	(21,700)	(600)
Less loans charged-off	(4,400)	(5,200)	(4,100)
Add back recoveries of loans previously charged-off	1,400	3,200	3,100
Ending balance	31,600	28,600	52,300
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				6,200	4,600	4,800
Collectively evaluated for impairment				25,400	24,000	47,500
Ending balance	28,600	52,300	53,900	31,600	28,600	52,300
Total loans:
Individually evaluated for impairment				58,300	68,600	66,500
Collectively evaluated for impairment				5,118,500	3,445,400	3,346,600
Total loans held for investment				5,176,800	3,514,000	3,413,100
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,700	5,100	5,000
Provision charged (credited) to operating expense	8,100	8,400	3,200
Less loans charged-off	(11,300)	(8,600)	(5,700)
Add back recoveries of loans previously charged-off	4,200	2,800	2,600
Ending balance	8,700	7,700	5,100
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				8,700	7,700	5,100
Ending balance	7,700	5,100	5,000	8,700	7,700	5,100
Total loans:
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				1,034,400	970,300	844,400
Total loans held for investment				1,034,400	970,300	844,400
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38,100	18,800	14,300
Provision charged (credited) to operating expense	(2,800)	21,900	4,400
Less loans charged-off	(6,800)	(5,800)	(1,700)
Add back recoveries of loans previously charged-off	2,100	3,200	1,800
Ending balance	30,600	38,100	18,800
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				4,400	9,300	6,500
Collectively evaluated for impairment				26,200	28,800	12,300
Ending balance	38,100	18,800	14,300	30,600	38,100	18,800
Total loans:
Individually evaluated for impairment				23,800	32,400	24,500
Collectively evaluated for impairment				1,191,600	765,500	767,900
Total loans held for investment				1,215,400	797,900	792,400
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,800	600	1,000
Provision charged (credited) to operating expense	(200)	1,400	(200)
Less loans charged-off	(400)	(200)	(200)
Add back recoveries of loans previously charged-off	0	0	0
Ending balance	1,200	1,800	600
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				200	100	300
Collectively evaluated for impairment				1,000	1,700	300
Ending balance	1,800	600	1,000	1,200	1,800	600
Total loans:
Individually evaluated for impairment				1,100	3,700	1,000
Collectively evaluated for impairment				135,100	129,200	141,200
Total loans held for investment				136,200	132,900	142,200
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0
Provision charged (credited) to operating expense	0	0	0
Less loans charged-off	0	0	0
Add back recoveries of loans previously charged-off	0	0	0
Ending balance	0	0	0
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				0	0	0
Ending balance	$ 0	$ 0	$ 0	0	0	0
Total loans:
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				4,900	1,600	1,300
Total loans held for investment				$ 4,900	$ 1,600	$ 1,300