Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial assets:
Investment securities held-to-maturity	$ 510,405	$ 513,273
Carrying Amount
Financial assets:
Cash and cash equivalents	882,834	782,023
Investment securities available-for-sale:	2,111,697	1,611,698
Investment securities held-to-maturity	505,985	512,770
Accrued interest receivable	40,621	29,852
Mortgage servicing rights, net	24,372	18,457
Loans held for sale	49,729	61,794
Net loans held for investment	7,427,815	5,340,536
Derivative assets	10,721	2,749
Deferred compensation plan assets	11,632	10,627
Total financial assets	11,065,406	8,370,506
Financial liabilities:
Total deposits, excluding time deposits	8,751,295	6,324,512
Time deposits	1,182,173	1,051,598
Securities sold under repurchase agreements	635,288	537,556
Other borrowed funds	30,008	6
Accrued interest payable	5,648	5,421
Long-term debt	8,039	27,970
Subordinated debentures held by subsidiary trusts	82,477	82,477
Derivative liabilities	9,099	1,314
Deferred compensation plan liabilities	11,632	10,627
Total financial liabilities	10,715,659	8,041,481
Estimated Fair Value
Financial assets:
Cash and cash equivalents	882,834	782,023
Investment securities available-for-sale:	2,111,697	1,611,698
Investment securities held-to-maturity	510,405	513,273
Accrued interest receivable	40,621	29,852
Mortgage servicing rights, net	38,142	35,656
Loans held for sale	49,729	61,794
Net loans held for investment	7,163,825	5,248,127
Derivative assets	10,721	2,749
Deferred compensation plan assets	11,632	10,627
Total financial assets	10,819,606	8,295,799
Financial liabilities:
Total deposits, excluding time deposits	8,751,295	6,324,512
Time deposits	1,172,783	1,044,670
Securities sold under repurchase agreements	635,288	537,556
Other borrowed funds	30,008	6
Accrued interest payable	5,648	5,421
Long-term debt	8,266	27,477
Subordinated debentures held by subsidiary trusts	92,303	73,554
Derivative liabilities	9,099	1,314
Deferred compensation plan liabilities	11,632	10,627
Total financial liabilities	10,716,322	8,025,137
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	882,834	782,023
Investment securities available-for-sale:	0	0
Investment securities held-to-maturity	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Loans held for sale	0	0
Net loans held for investment	0	0
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	882,834	782,023
Financial liabilities:
Total deposits, excluding time deposits	8,751,295	6,324,512
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liabilities	0	0
Deferred compensation plan liabilities	0	0
Total financial liabilities	8,751,295	6,324,512
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale:	2,111,697	1,611,698
Investment securities held-to-maturity	510,405	513,273
Accrued interest receivable	40,621	29,852
Mortgage servicing rights, net	38,142	35,656
Loans held for sale	49,729	61,794
Net loans held for investment	7,129,336	5,208,783
Derivative assets	10,721	2,749
Deferred compensation plan assets	11,632	10,627
Total financial assets	9,902,283	7,474,432
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,172,783	1,044,670
Securities sold under repurchase agreements	635,288	537,556
Other borrowed funds	30,008	6
Accrued interest payable	5,648	5,421
Long-term debt	8,266	27,477
Subordinated debentures held by subsidiary trusts	92,303	73,554
Derivative liabilities	9,099	1,314
Deferred compensation plan liabilities	11,632	10,627
Total financial liabilities	1,965,027	1,700,625
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale:	0	0
Investment securities held-to-maturity	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Loans held for sale	0	0
Net loans held for investment		39,344
Impaired loans	34,489
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	34,489	39,344
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liabilities	0	0
Deferred compensation plan liabilities	0	0
Total financial liabilities	$ 0	$ 0