Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held to maturity, Estimated fair value, Less than 12 months	$ 2,839	$ 4,967
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(6)	(596)
Held to maturity, Estimated fair value, 12 months or more	157,711	168,569
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(92,989)	(98,572)
Estimated fair value, Less than 12 months	97,059	421,016
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(4,338)	(29,096)
Estimated fair value, 12 months or more	949,352	851,849
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(182,593)	(264,768)
Total investment securities, fair value less than 12 months	99,898	425,983
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses	(4,344)	(29,692)
Total of investment securities, fair value, more than 12 months	1,107,063	1,020,418
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses	(275,582)	(363,340)
Privately issued mortgage-backed securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held to maturity, Estimated fair value, Less than 12 months		1,883
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses		(592)
Held to maturity, Estimated fair value, 12 months or more	155,189	167,139
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(92,966)	(98,548)
Obligations of states and political subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held to maturity, Estimated fair value, Less than 12 months	2,839	3,084
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(6)	(4)
Held to maturity, Estimated fair value, 12 months or more	2,522	1,430
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(23)	(24)
Estimated fair value, Less than 12 months	979
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(6)
Estimated fair value, 12 months or more	685	1,228
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(4)	(20)
Mortgage-backed securities [Member] | Government issued or guaranteed [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	5,838	38,492
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(28)	(190)
Estimated fair value, 12 months or more	9,319	6,017
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(147)	(85)
Mortgage-backed securities [Member] | Privately issued residential [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	81,620	297,133
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(454)	(14,188)
Estimated fair value, 12 months or more	828,916	751,077
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(148,303)	(225,300)
Mortgage-backed securities [Member] | Privately issued commercial [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, 12 months or more	9,744	15,029
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(269)	(2,650)
Collateralized debt obligations [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	3,136	2,871
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(30)	(335)
Estimated fair value, 12 months or more	5,842	4,863
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(1,210)	(2,153)
Other debt securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months		72,637
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses		(9,883)
Estimated fair value, 12 months or more	93,266	73,635
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(29,159)	(34,560)
Equity securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	5,486	9,883
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(3,820)	(4,500)
Estimated fair value, 12 months or more	1,580
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	$ (3,501)