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Investment securities - Investment Ratings of All Privately Issued Mortgage-Backed Securities, Collateralized Debt Obligations and Other Debt Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Amortized Cost $ 1,777,429
Total securities with disclosed rating, Estimated Fair Value 1,566,422
Obligations of states and political subdivisions [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Amortized Cost 202,478
Total securities with disclosed rating, Estimated Fair Value 210,624
Privately issued residential [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Amortized Cost 1,376,017
Total securities with disclosed rating, Estimated Fair Value 1,160,524
Privately issued commercial [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Amortized Cost 8,648
Total securities with disclosed rating, Estimated Fair Value 11,000
Collateralized debt obligations [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Amortized Cost 43,228
Total securities with disclosed rating, Estimated Fair Value 61,869
Other debt securities [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Amortized Cost 147,058
Total securities with disclosed rating, Estimated Fair Value 122,405
A or Better [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 392,259
A or Better [Member] | Obligations of states and political subdivisions [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 169,660
A or Better [Member] | Privately issued residential [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 206,479
A or Better [Member] | Collateralized debt obligations [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 4,268
A or Better [Member] | Other debt securities [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 11,852
BBB [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 162,766
BBB [Member] | Obligations of states and political subdivisions [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 7,215
BBB [Member] | Privately issued residential [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 87,866
BBB [Member] | Collateralized debt obligations [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 10,298
BBB [Member] | Other debt securities [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 57,387
BB [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 189,979
BB [Member] | Obligations of states and political subdivisions [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 200
BB [Member] | Privately issued residential [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 151,980
BB [Member] | Collateralized debt obligations [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 10,770
BB [Member] | Other debt securities [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 27,029
B or Less [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 774,830
B or Less [Member] | Privately issued residential [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 712,765
B or Less [Member] | Privately issued commercial [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 11,000
B or Less [Member] | Collateralized debt obligations [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 36,533
B or Less [Member] | Other debt securities [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 14,532
Not Rated [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 46,588
Not Rated [Member] | Obligations of states and political subdivisions [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 33,549
Not Rated [Member] | Privately issued residential [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value 1,434
Not Rated [Member] | Other debt securities [Member]
 
Schedule of Available-for-sale Securities [Line Items]  
Total securities with disclosed rating, Estimated Fair Value $ 11,605