Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Investment debt securities held-to-maturity	$ 118,000	$ 400,700
Derivative assets	14,600	10,100
Financial liabilities:
Derivative liabilities	13,500	9,400
Carrying Amount
Financial assets:
Cash and cash equivalents	955,200	822,000
Investment debt securities available-for-sale:	2,572,000	2,270,700
Investment debt securities held-to-maturity	117,000	406,800
Accrued interest receivable	46,500	44,900
Mortgage servicing rights, net	27,900	27,700
Loans Held-for-sale, Fair Value Disclosure	34,000	33,300
Net loans held for investment	8,386,800	8,397,400
Derivative assets	14,600	10,100
Deferred Compensation Plan Assets	12,400	12,100
Total financial assets	12,166,400	12,025,000
Financial liabilities:
Total deposits, excluding time deposits	9,408,300	9,363,700
Time deposits	1,406,400	1,317,000
Securities sold under repurchase agreements	672,600	712,400
Accrued interest payable	10,000	7,800
Long-term debt	15,800	15,800
Subordinated debentures held by subsidiary trusts	86,900	86,900
Derivative liabilities	13,500	9,400
Deferred Compensation Liability, Current and Noncurrent	12,400	12,100
Total financial liabilities	11,625,900	11,525,100
Estimated Fair Value
Financial assets:
Cash and cash equivalents	955,200	822,000
Investment debt securities available-for-sale:	2,572,000	2,270,700
Investment debt securities held-to-maturity	118,000	400,700
Accrued interest receivable	46,500	44,900
Mortgage servicing rights, net	40,500	42,400
Loans held for sale	34,000	33,300
Net loans held for investment	8,408,600	8,439,700
Derivative assets	14,600	10,100
Deferred Compensation Plan Assets	12,400	12,100
Total financial assets	12,201,800	12,075,900
Financial liabilities:
Total deposits, excluding time deposits	9,408,300	9,363,700
Time deposits	1,393,200	1,299,000
Securities sold under repurchase agreements	672,600	712,400
Accrued interest payable	10,000	7,800
Long-term debt	13,900	13,000
Subordinated debentures held by subsidiary trusts	85,100	84,900
Derivative liabilities	13,500	9,400
Deferred Compensation Liability, Current and Noncurrent	12,400	12,100
Total financial liabilities	11,609,000	11,502,300
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	955,200	822,000
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	955,200	822,000
Financial liabilities:
Total deposits, excluding time deposits	9,408,300	9,363,700
Time deposits	0	0
Securities sold under repurchase agreements	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 Liability, Current and Noncurrent	0	0
Total financial liabilities	9,408,300	9,363,700
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment debt securities available-for-sale:	2,572,000	2,270,700
Investment debt securities held-to-maturity	118,000	400,700
Accrued interest receivable	46,500	44,900
Mortgage servicing rights, net	40,500	42,400
Loans held for sale	34,000	33,300
Net loans held for investment	8,377,700	8,415,600
Derivative assets	14,600	10,100
Deferred Compensation Plan Assets	12,400	12,100
Total financial assets	11,215,700	11,229,800
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,393,200	1,299,000
Securities sold under repurchase agreements	672,600	712,400
Accrued interest payable	10,000	7,800
Long-term debt	13,900	13,000
Subordinated debentures held by subsidiary trusts	85,100	84,900
Derivative liabilities	13,500	9,400
Deferred Compensation Liability, Current and Noncurrent	12,400	12,100
Total financial liabilities	2,200,700	2,138,600
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	30,900	24,100
Derivative assets	0	0
Deferred Compensation Plan Assets	0	0
Total financial assets	30,900	24,100
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	0	0
Securities sold under repurchase agreements	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 Liability, Current and Noncurrent	0	0
Total financial liabilities	$ 0	$ 0