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Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Amortized Cost $ 3,082,082 $ 3,139,748
Held to Maturity, Gross Unrealized Gains 30,556 27,919
Held to Maturity, Gross Unrealized Losses 67,179 138,004
Held to Maturity, Estimated Fair Value 3,045,459 3,029,663
Available for Sale, Amortized Cost 5,364,733 5,799,135
Available for Sale, Gross Unrealized Gains 122,411 105,276
Available for Sale, Gross Unrealized Losses 4,679 8,975
Available for Sale, Estimated Fair Value 5,482,465 5,895,436
U.S. Treasury [Member]
   
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Amortized Cost 248,693 248,592
Held to Maturity, Gross Unrealized Gains 18,475 20,139
Held to Maturity, Gross Unrealized Losses 0 0
Held to Maturity, Estimated Fair Value 267,168 268,731
Available for Sale, Amortized Cost 1,825,033 2,522,159
Available for Sale, Gross Unrealized Gains 15,178 18,395
Available for Sale, Gross Unrealized Losses 1,453 0
Available for Sale, Estimated Fair Value 1,838,758 2,540,554
US Government Corporations and Agencies Securities [Member]
   
Schedule of Available-for-Sale Securities [Line Items]    
Available for Sale, Amortized Cost 0 54,024
Available for Sale, Gross Unrealized Gains 0 0
Available for Sale, Gross Unrealized Losses 0 44
Available for Sale, Estimated Fair Value 0 53,980
Residential mortgage- backed securities [Member]
   
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Amortized Cost 9,512 9,674
Held to Maturity, Gross Unrealized Gains 92 89
Held to Maturity, Gross Unrealized Losses 58 143
Held to Maturity, Estimated Fair Value 9,546 9,620
Available for Sale, Amortized Cost 1,610,400 1,710,664
Available for Sale, Gross Unrealized Gains 68,553 66,791
Available for Sale, Gross Unrealized Losses 1,036 1,439
Available for Sale, Estimated Fair Value 1,677,917 1,776,016
States and political subdivisions [Member]
   
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Amortized Cost 2,822,877 2,880,482
Held to Maturity, Gross Unrealized Gains 11,989 7,691
Held to Maturity, Gross Unrealized Losses 67,121 137,861
Held to Maturity, Estimated Fair Value 2,767,745 2,750,312
Available for Sale, Amortized Cost 1,893,310 1,476,316
Available for Sale, Gross Unrealized Gains 38,680 20,090
Available for Sale, Gross Unrealized Losses 2,190 7,492
Available for Sale, Estimated Fair Value 1,929,800 1,488,914
Other [Member]
   
Schedule of Available-for-Sale Securities [Line Items]    
Held to Maturity, Amortized Cost 1,000 1,000
Held to Maturity, Gross Unrealized Gains 0 0
Held to Maturity, Gross Unrealized Losses 0 0
Held to Maturity, Estimated Fair Value 1,000 1,000
Available for Sale, Amortized Cost 35,990 35,972
Available for Sale, Gross Unrealized Gains 0 0
Available for Sale, Gross Unrealized Losses 0 0
Available for Sale, Estimated Fair Value $ 35,990 $ 35,972