Fair Value - Estimated fair value and related carrying amount, of the Company’s financial instruments - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial assets:
Loans held for sale	$ 70,890	$ 50,572
Financial liabilities:
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	286,857	313,773
Increase (Decrease) in Loans Held-for-sale	636,836	690,495	$ 318,936
Other borrowings	216,385	55,358
Carrying Amount
Financial assets:
Cash and cash equivalents	377,627	374,448
Investment securities	1,673,769	1,014,981
Loans held for sale	70,890	50,572
Loans, net of allowance for loan losses	10,575,417	6,643,326
Accrued interest receivable	32,727	18,618
Mortgage servicing rights	31,119	29,037
Interest rate swap - non-designated hedge	2,367	203
Other derivative financial instruments (Mortgage - banking related)	939	2,403
Financial liabilities:
Deposits	11,532,766	7,334,423
Federal funds purchased and securities sold under agreements to repurchase	286,857	313,773
Other borrowings	216,385	55,358
Accrued interest payable	2,789	1,359
Interest rate swap - non-designated hedge	2,750	181
Interest rate swap - cash flow hedge	246	498
Other derivative financial instruments (Mortgage banking related)	252	51
Fair Value.
Financial assets:
Cash and cash equivalents	377,627	374,448
Investment securities	1,673,796	1,015,137
Loans held for sale	70,890	50,572
Loans, net of allowance for loan losses	10,724,264	6,649,575
Accrued interest receivable	32,727	18,618
Mortgage servicing rights	31,119	29,037
Interest rate swap - non-designated hedge	2,367	203
Other derivative financial instruments (Mortgage - banking related)	939	2,403
Financial liabilities:
Deposits	10,796,380	6,935,867
Federal funds purchased and securities sold under agreements to repurchase	286,857	313,773
Other borrowings	219,421	54,379
Accrued interest payable	2,789	1,359
Interest rate swap - non-designated hedge	2,750	181
Interest rate swap - cash flow hedge	246	498
Other derivative financial instruments (Mortgage banking related)	252	51
Commitments to extend credit | Fair Value.
Financial liabilities:
Commitments to extend credit	41,319	1,587
Quoted Prices In Active Markets for Identical Assets (Level 1) | Fair Value.
Financial assets:
Cash and cash equivalents	377,627	374,448
Investment securities	20,530	12,041
Significant Other Observable Inputs (Level 2) | Fair Value.
Financial assets:
Investment securities	1,653,266	1,003,096
Loans held for sale	70,890	50,572
Accrued interest receivable	7,051	3,642
Interest rate swap - non-designated hedge	2,367	203
Other derivative financial instruments (Mortgage - banking related)	939	2,403
Financial liabilities:
Deposits	10,796,380	6,935,867
Federal funds purchased and securities sold under agreements to repurchase	286,857	313,773
Other borrowings	219,421	54,379
Accrued interest payable	2,789	1,359
Interest rate swap - non-designated hedge	2,750	181
Interest rate swap - cash flow hedge	246	498
Other derivative financial instruments (Mortgage banking related)	252	51
Significant Other Observable Inputs (Level 2) | Commitments to extend credit | Fair Value.
Financial liabilities:
Commitments to extend credit	41,319	1,587
Significant Unobservable Inputs (Level 3) | Fair Value.
Financial assets:
Loans, net of allowance for loan losses	10,724,264	6,649,575
Accrued interest receivable	25,676	14,976
Mortgage servicing rights	31,119	29,037
Recurring basis
Financial assets:
Loans held for sale	70,890	50,572
Mortgage servicing rights	31,119	29,037
Recurring basis | Significant Other Observable Inputs (Level 2)
Financial assets:
Loans held for sale	70,890	50,572
Recurring basis | Significant Unobservable Inputs (Level 3)
Financial assets:
Mortgage servicing rights	$ 31,119	$ 29,037