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Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Schedule of Available-for-Sale Securities [Line Items]  
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 916,127
Held to Maturity, Less than 12 Months, Unrealized Losses 20,193
Held to Maturity, More than 12 Months, Estimated Fair Value 1,338,645
Held to Maturity, More than 12 Months, Unrealized Losses 46,986
Held to Maturity, Total Estimated Fair Value 2,254,772
Held to Maturity, Total Unrealized Losses 67,179
Available for Sale, Less than 12 Months, Estimated Fair Value 648,406
Available for Sale, Less than 12 Months, Unrealized Losses 4,447
Available for Sale, More than 12 Months, Estimated Fair Value 2,745
Available for Sale, More than 12 Months, Unrealized Losses 232
Available for Sale, Total Estimated Fair Value 651,151
Available for Sale, Total Unrealized Losses 4,679
Residential mortgage- backed securities [Member]
  
Schedule of Available-for-Sale Securities [Line Items]  
Held to Maturity, Less than 12 Months, Estimated Fair Value 6,967
Held to Maturity, Less than 12 Months, Unrealized Losses 58
Held to Maturity, More than 12 Months, Estimated Fair Value 0
Held to Maturity, More than 12 Months, Unrealized Losses 0
Held to Maturity, Total Estimated Fair Value 6,967
Held to Maturity, Total Unrealized Losses 58
Available for Sale, Less than 12 Months, Estimated Fair Value 14,598
Available for Sale, Less than 12 Months, Unrealized Losses 804
Available for Sale, More than 12 Months, Estimated Fair Value 2,745
Available for Sale, More than 12 Months, Unrealized Losses 232
Available for Sale, Total Estimated Fair Value 17,343
Available for Sale, Total Unrealized Losses 1,036
States and political subdivisions [Member]
  
Schedule of Available-for-Sale Securities [Line Items]  
Held to Maturity, Less than 12 Months, Estimated Fair Value 909,160
Held to Maturity, Less than 12 Months, Unrealized Losses 20,135
Held to Maturity, More than 12 Months, Estimated Fair Value 1,338,645
Held to Maturity, More than 12 Months, Unrealized Losses 46,986
Held to Maturity, Total Estimated Fair Value 2,247,805
Held to Maturity, Total Unrealized Losses 67,121
Available for Sale, Less than 12 Months, Estimated Fair Value 332,824
Available for Sale, Less than 12 Months, Unrealized Losses 2,190
Available for Sale, More than 12 Months, Estimated Fair Value 0
Available for Sale, More than 12 Months, Unrealized Losses 0
Available for Sale, Total Estimated Fair Value 332,824
Available for Sale, Total Unrealized Losses 2,190
U.S. Treasury [Member]
  
Schedule of Available-for-Sale Securities [Line Items]  
Available for Sale, Less than 12 Months, Estimated Fair Value 300,984
Available for Sale, Less than 12 Months, Unrealized Losses 1,453
Available for Sale, More than 12 Months, Estimated Fair Value 0
Available for Sale, More than 12 Months, Unrealized Losses 0
Available for Sale, Total Estimated Fair Value 300,984
Available for Sale, Total Unrealized Losses $ 1,453