Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Investment debt securities held-to-maturity	$ 407,400	$ 483,300
Carrying Amount
Financial assets:
Cash and cash equivalents	965,000	759,000
Investment debt securities available-for-sale:	2,158,800	2,208,700
Investment debt securities held-to-maturity	417,800	484,500
Accrued interest receivable	48,000	38,000
Mortgage servicing rights, net	27,000	24,800
Loans held for sale	37,700
Net loans held for investment	8,480,300	7,542,200
Derivative assets	12,400	8,800
Deferred compensation plan assets	13,000	12,200
Total financial assets	12,160,000	11,078,200
Financial liabilities:
Total deposits, excluding time deposits	9,560,300	8,783,000
Time deposits	1,285,300	1,151,900
Securities sold under repurchase agreements	635,900	643,000
Other borrowed funds	1,500	20,000
Accrued interest payable	6,700	5,600
Long-term debt	22,400	13,100
Subordinated debentures held by subsidiary trusts	86,900	82,500
Derivative liabilities	11,300	7,900
Deferred compensation plan liabilities	13,000	12,200
Total financial liabilities	11,623,300	10,719,200
Estimated Fair Value
Financial assets:
Cash and cash equivalents	965,000	759,000
Investment debt securities available-for-sale:	2,158,800	2,208,700
Investment debt securities held-to-maturity	407,400	483,300
Accrued interest receivable	48,000	38,000
Mortgage servicing rights, net	41,800	40,100
Loans held for sale	37,700
Net loans held for investment	8,406,800	7,298,800
Derivative assets	12,400	8,800
Deferred compensation plan assets	13,000	12,200
Total financial assets	12,090,900	10,848,900
Financial liabilities:
Total deposits, excluding time deposits	9,560,300	8,783,000
Time deposits	1,269,200	1,137,900
Securities sold under repurchase agreements	635,900	643,000
Other borrowed funds	1,500	20,000
Accrued interest payable	6,700	5,600
Long-term debt	19,600	11,300
Subordinated debentures held by subsidiary trusts	82,400	76,700
Derivative liabilities	11,300	7,900
Deferred compensation plan liabilities	13,000	12,200
Total financial liabilities	11,599,900	10,697,600
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	965,000	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
Net loans held for investment	0	0
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	965,000	759,000
Financial liabilities:
Total deposits, excluding time deposits	9,560,300	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	9,560,300	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:	2,158,800	2,208,700
Investment debt securities held-to-maturity	407,400	483,300
Accrued interest receivable	48,000	38,000
Mortgage servicing rights, net	41,800	40,100
Loans held for sale	37,700
Net loans held for investment	8,378,100	7,266,200
Derivative assets	12,400	8,800
Deferred compensation plan assets	13,000	12,200
Total financial assets	11,097,200	10,057,300
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,269,200	1,137,900
Securities sold under repurchase agreements	635,900	643,000
Other borrowed funds	1,500	20,000
Accrued interest payable	6,700	5,600
Long-term debt	19,600	11,300
Subordinated debentures held by subsidiary trusts	82,400	76,700
Derivative liabilities	11,300	7,900
Deferred compensation plan liabilities	13,000	12,200
Total financial liabilities	2,039,600	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
Net loans held for investment		32,600
Impaired loans	28,700
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	28,700	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