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Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]  
Held to Maturity, Less than 12 Months, Estimated Fair Value $ 2,176,890
Held to Maturity, Less than 12 Months, Unrealized Losses 30,863
Held to Maturity, More than 12 Months, Estimated Fair Value   
Held to Maturity, More than 12 Months, Unrealized Losses   
Held to Maturity, Total Estimated Fair Value 2,176,890
Held to Maturity, Total Unrealized Losses 30,863
Available for Sale, Less than 12 Months, Estimated Fair Value 12,586
Available for Sale, Less than 12 Months, Unrealized Losses 153
Available for Sale, More than 12 Months, Estimated Fair Value   
Available for Sale, More than 12 Months, Unrealized Losses   
Available for Sale, Total Estimated Fair Value 12,586
Available for Sale, Total Unrealized Losses 153
Residential Mortgage Backed Securities [Member]
 
Schedule of Available-for-Sale Securities [Line Items]  
Available for Sale, Less than 12 Months, Estimated Fair Value 7,668
Available for Sale, Less than 12 Months, Unrealized Losses 108
Available for Sale, More than 12 Months, Estimated Fair Value   
Available for Sale, More than 12 Months, Unrealized Losses   
Available for Sale, Total Estimated Fair Value 7,668
Available for Sale, Total Unrealized Losses 108
States and Political Subdivisions [Member]
 
Schedule of Available-for-Sale Securities [Line Items]  
Held to Maturity, Less than 12 Months, Estimated Fair Value 2,175,897
Held to Maturity, Less than 12 Months, Unrealized Losses 30,856
Held to Maturity, More than 12 Months, Estimated Fair Value   
Held to Maturity, More than 12 Months, Unrealized Losses   
Held to Maturity, Total Estimated Fair Value 2,175,897
Held to Maturity, Total Unrealized Losses 30,856
Available for Sale, Less than 12 Months, Estimated Fair Value 4,918
Available for Sale, Less than 12 Months, Unrealized Losses 45
Available for Sale, More than 12 Months, Estimated Fair Value   
Available for Sale, More than 12 Months, Unrealized Losses   
Available for Sale, Total Estimated Fair Value 4,918
Available for Sale, Total Unrealized Losses 45
Other [Member]
 
Schedule of Available-for-Sale Securities [Line Items]  
Held to Maturity, Less than 12 Months, Estimated Fair Value 993
Held to Maturity, Less than 12 Months, Unrealized Losses 7
Held to Maturity, More than 12 Months, Estimated Fair Value   
Held to Maturity, More than 12 Months, Unrealized Losses   
Held to Maturity, Total Estimated Fair Value 993
Held to Maturity, Total Unrealized Losses $ 7