Fair Value - Estimated fair value and related carrying amount, of the Company’s financial instruments - (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financial assets:
Loans held for sale	$ 42,690	$ 46,988	$ 70,890
Financial liabilities:
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	357,574	352,431	286,857
Increase (Decrease) in Loans Held-for-sale	10,461	176,946
Other borrowings	215,589	107,988	216,385
Carrying Amount
Financial assets:
Cash and cash equivalents	644,504	663,126	377,627
Investment securities	1,665,590	1,400,609	1,673,769
Loans held for sale	42,690	46,988	70,890
Loans, net of allowance for loan losses	10,589,521	7,868,840	10,575,417
Interest Receivable	31,175	22,823	32,727
Mortgage servicing rights	34,196	30,063	31,119
Interest rate swap - non-designated hedge	5,755	133	2,367
Other derivative financial instruments (Mortgage - banking related)	1,665	2,280	939
Financial liabilities:
Deposits	11,663,098	9,033,438	11,532,766
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	357,574	352,431	286,857
Other borrowings	215,589	107,988	216,385
Interest Payable	3,563	1,452	2,789
Interest rate swap - non-designated hedge	5,932	426	2,750
Interest rate swap - cash flow hedge	162	121	246
Other derivative financial instruments (Mortgage banking related)		387	252
Fair Value.
Financial assets:
Cash and cash equivalents	644,504	663,126	377,627
Investment securities	1,665,600	1,400,731	1,673,796
Loans held for sale	42,690	46,988	70,890
Loans, net of allowance for loan losses	10,419,134	7,936,291	10,724,264
Interest Receivable	31,175	22,823	32,727
Mortgage servicing rights	34,196	30,063	31,119
Interest rate swap - non-designated hedge	5,755	133	2,367
Other derivative financial instruments (Mortgage - banking related)	1,665	2,280	939
Financial liabilities:
Deposits	10,780,965	8,462,144	10,796,380
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	357,574	352,431	286,857
Other borrowings	219,037	109,470	219,421
Interest Payable	3,563	1,452	2,789
Interest rate swap - non-designated hedge	5,932	426	2,750
Interest rate swap - cash flow hedge	162	121	246
Other derivative financial instruments (Mortgage banking related)		387	252
Commitments to extend credit | Fair Value.
Financial liabilities:
Commitments to extend credit	(48,841)		41,319
Standby letters of credit and financial guarantees | Fair Value.
Financial liabilities:
Commitments to extend credit		16,756
Quoted Prices In Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	644,504	663,126	377,627
Investment securities	23,479	16,267	20,530
Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities	1,642,121	1,384,464	1,653,266
Loans held for sale	42,690	46,988	70,890
Interest Receivable	6,661	5,558	7,051
Interest rate swap - non-designated hedge	5,755	133	2,367
Other derivative financial instruments (Mortgage - banking related)	1,665	2,280	939
Financial liabilities:
Deposits	10,780,965	8,462,144	10,796,380
Federal Funds Purchased and Securities Sold under Agreements to Repurchase.	357,574	352,431	286,857
Other borrowings	219,037	109,470	219,421
Interest Payable	3,563	1,452	2,789
Interest rate swap - non-designated hedge	5,932	426	2,750
Interest rate swap - cash flow hedge	162	121	246
Other derivative financial instruments (Mortgage banking related)		387	252
Significant Other Observable Inputs (Level 2) | Commitments to extend credit
Financial liabilities:
Commitments to extend credit	(48,841)		41,319
Significant Other Observable Inputs (Level 2) | Standby letters of credit and financial guarantees
Financial liabilities:
Commitments to extend credit		16,756
Significant Unobservable Inputs (Level 3)
Financial assets:
Loans, net of allowance for loan losses	10,419,134	7,936,291	10,724,264
Interest Receivable	24,514	17,265	25,676
Mortgage servicing rights	34,196	30,063	31,119
Recurring basis
Financial assets:
Loans held for sale	42,690	46,988	70,890
Mortgage servicing rights	34,196	30,063	31,119
Recurring basis | Significant Other Observable Inputs (Level 2)
Financial assets:
Loans held for sale	42,690	46,988	70,890
Recurring basis | Significant Unobservable Inputs (Level 3)
Financial assets:
Mortgage servicing rights	$ 34,196	$ 30,063	$ 31,119