Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Held-to-maturity investment securities, estimated fair value	$ 483,349	$ 513,273
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Financial liabilities:
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Carrying Amount
Financial assets:
Cash and cash equivalents	759,000	782,000
Investment securities available-for-sale	2,208,700	1,611,700
Held-to-maturity investment securities, estimated fair value	484,500	512,800
Accrued interest receivable	38,000	29,900
Mortgage servicing rights, net	24,800	18,500
Net loans	7,542,200	5,402,300
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Total financial assets	11,078,200	8,370,500
Financial liabilities:
Total deposits, excluding time deposits	8,783,000	6,324,500
Time deposits	1,151,900	1,051,600
Securities sold under repurchase agreements	643,000	537,600
Other borrowed funds	20,000	0
Accrued interest payable	5,600	5,400
Long-term debt	13,100	28,000
Subordinated debentures held by subsidiary trusts	82,500	82,500
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Total financial liabilities	10,719,200	8,041,500
Estimated Fair Value
Financial assets:
Cash and cash equivalents	759,000	782,000
Investment securities available-for-sale	2,208,700	1,611,700
Held-to-maturity investment securities, estimated fair value	483,300	513,300
Accrued interest receivable	38,000	29,900
Mortgage servicing rights, net	40,100	35,700
Net loans	7,298,800	5,309,900
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Total financial assets	10,848,900	8,295,800
Financial liabilities:
Total deposits, excluding time deposits	8,783,000	6,324,500
Time deposits	1,137,900	1,044,700
Securities sold under repurchase agreements	643,000	537,600
Other borrowed funds	20,000	0
Accrued interest payable	5,600	5,400
Long-term debt	11,300	27,500
Subordinated debentures held by subsidiary trusts	76,700	73,600
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Total financial liabilities	10,697,600	8,025,200
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	759,000	782,000
Investment securities available-for-sale	0	0
Held-to-maturity investment securities, estimated fair value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Net loans	0	0
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	759,000	782,000
Financial liabilities:
Total deposits, excluding time deposits	8,783,000	6,324,500
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	8,783,000	6,324,500
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	2,208,700	1,611,700
Held-to-maturity investment securities, estimated fair value	483,300	513,300
Accrued interest receivable	38,000	29,900
Mortgage servicing rights, net	40,100	35,700
Net loans	7,266,200	5,270,600
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Total financial assets	10,057,300	7,474,500
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,137,900	1,044,700
Securities sold under repurchase agreements	643,000	537,600
Other borrowed funds	20,000	0
Accrued interest payable	5,600	5,400
Long-term debt	11,300	27,500
Subordinated debentures held by subsidiary trusts	76,700	73,600
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Total financial liabilities	1,914,600	1,700,700
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	0	0
Held-to-maturity investment securities, estimated fair value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Net loans	32,600	39,300
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	32,600	39,300
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