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