Fair Value Measurements - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 6,033,370	$ 6,203,299
Trading account securities	16,777	30,074
Derivative assets	14,360	2,246
Derivative liabilities	49,137	74,159
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	3,042,723	3,057,921
Trading account securities	14,489	14,038
Residential mortgage- backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,075,573	2,518,003
States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	879,175	591,483
Trading account securities	2,288	16,036
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	35,899	35,892
Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	48,913	60,890
Derivative liabilities	55,075	72,557
Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	6,655	3,867
Derivative liabilities	6,433	3,772
Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	590	3
Derivative liabilities	567
Level 1 [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	3,042,723	3,057,921
Trading account securities	14,489	14,038
Level 1 [Member] | Foreign currency forward contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	590	3
Derivative liabilities	567
Level 2 [Member] | Residential mortgage- backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,075,573	2,518,003
Level 2 [Member] | States and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	879,175	591,483
Trading account securities	2,288	16,036
Level 2 [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	35,899	35,892
Level 2 [Member] | Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	48,677	60,535
Derivative liabilities	55,075	72,557
Level 2 [Member] | Commodity swaps and options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	6,655	3,867
Derivative liabilities	6,433	3,772
Level 3 [Member] | Interest rate swaps, caps and floors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 236	$ 355