Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	$ 931,600	$ 1,154,700
Securities available for sale	6,771,179	6,806,949
Securities held to maturity	880,600	667,106
Trading securities	56,311	29,617
Other securities	66,300	65,200
Loans (exclusive of allowance for loan loss)	10,093,800	9,431,300
Derivatives	23,700	12,300
Demand and savings deposits	14,503,900	13,836,900
Time deposits	1,144,800	1,255,900
Other borrowings	1,793,600	1,823,100
Long-term debt	85,320	86,070
Derivatives	31,900	12,300
Total Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	931,600	1,154,700
Securities available for sale	6,771,200	6,806,900
Securities held to maturity	991,700	691,400
Trading securities	56,300	29,600
Other securities	66,300	65,200
Loans (exclusive of allowance for loan loss)	10,196,500	9,452,100
Derivatives	23,700	12,300
Demand and savings deposits	14,503,900	13,836,900
Time deposits	1,144,800	1,255,900
Other borrowings	1,793,600	1,823,100
Long-term debt	85,900	86,400
Derivatives	31,900	12,300
Commitments to extend credit for loans	2,800	4,900
Commercial letters of credit	100	300
Standby letters of credit	1,200	2,600
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	744,900	997,000
Securities available for sale	443,400	429,700
Trading securities	28,200	18,100
Demand and savings deposits	14,503,900	13,836,900
Other borrowings	247,900	66,900
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	186,700	157,700
Securities available for sale	6,327,800	6,377,200
Securities held to maturity	991,700	691,400
Trading securities	28,100	11,500
Other securities	66,300	65,200
Loans (exclusive of allowance for loan loss)	10,196,500	9,452,100
Derivatives	23,700	12,300
Time deposits	1,144,800	1,255,900
Other borrowings	1,545,700	1,756,200
Long-term debt	85,900	86,400
Derivatives	$ 31,900	$ 12,300