Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial assets:
Investment debt securities held-to-maturity	$ 455,500	$ 483,300
Carrying Amount
Financial assets:
Cash and cash equivalents	744,200	759,000
Investment debt securities available-for-sale:	2,280,700	2,208,700
Investment debt securities held-to-maturity	462,300	484,500
Accrued interest receivable	38,900	38,000
Mortgage servicing rights, net	25,200	24,800
Loans held for sale	32,300	0
Net loans held for investment	7,614,500	7,542,200
Derivative assets	9,700	8,800
Deferred compensation plan assets	12,100	12,200
Total financial assets	11,219,900	11,078,200
Financial liabilities:
Total deposits, excluding time deposits	8,891,200	8,783,000
Time deposits	1,134,700	1,151,900
Securities sold under repurchase agreements	633,800	643,000
Other borrowed funds	0	20,000
Accrued interest payable	75,700	5,600
Long-term debt	15,700	13,100
Subordinated debentures held by subsidiary trusts	82,500	82,500
Derivative liabilities	7,900	7,900
Deferred compensation plan liabilities	12,100	12,200
Total financial liabilities	10,853,600	10,719,200
Estimated Fair Value
Financial assets:
Cash and cash equivalents	744,200	759,000
Investment debt securities available-for-sale:	2,280,700	2,208,700
Investment debt securities held-to-maturity	455,500	483,300
Accrued interest receivable	38,900	38,000
Mortgage servicing rights, net	43,700	40,100
Loans held for sale	32,300	0
Net loans held for investment	7,388,200	7,298,800
Derivative assets	9,700	8,800
Deferred compensation plan assets	12,100	12,200
Total financial assets	11,005,300	10,848,900
Financial liabilities:
Total deposits, excluding time deposits	8,891,200	8,783,000
Time deposits	1,116,800	1,137,900
Securities sold under repurchase agreements	633,800	643,000
Other borrowed funds	0	20,000
Accrued interest payable	75,700	5,600
Long-term debt	15,800	11,300
Subordinated debentures held by subsidiary trusts	76,900	76,700
Derivative liabilities	7,900	7,900
Deferred compensation plan liabilities	12,100	12,200
Total financial liabilities	10,830,200	10,697,600
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	744,200	759,000
Investment debt securities available-for-sale:	0	0
Investment debt 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	744,200	759,000
Financial liabilities:
Total deposits, excluding time deposits	8,891,200	8,783,000
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	8,891,200	8,783,000
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment debt securities available-for-sale:	2,280,700	2,208,700
Investment debt securities held-to-maturity	455,500	483,300
Accrued interest receivable	38,900	38,000
Mortgage servicing rights, net	43,700	40,100
Loans held for sale	32,300	0
Net loans held for investment	7,352,200	7,266,200
Derivative assets	9,700	8,800
Deferred compensation plan assets	12,100	12,200
Total financial assets	10,225,100	10,057,300
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,116,800	1,137,900
Securities sold under repurchase agreements	633,800	643,000
Other borrowed funds	0	20,000
Accrued interest payable	75,700	5,600
Long-term debt	15,800	11,300
Subordinated debentures held by subsidiary trusts	76,900	76,700
Derivative liabilities	7,900	7,900
Deferred compensation plan liabilities	12,100	12,200
Total financial liabilities	1,939,000	1,914,600
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment debt securities available-for-sale:	0	0
Investment debt 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		32,600
Impaired loans	36,000
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	36,000	32,600
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