Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	$ 139,295	$ 421,016
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(2,053)	(29,096)
Estimated fair value, 12 months or more	888,499	851,849
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(155,199)	(264,768)
Held to maturity, Estimated fair value, Less than 12 months	1,026	4,967
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(5)	(596)
Held to maturity, Estimated fair value, 12 months or more	150,831	168,569
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(94,922)	(98,572)
Total investment securities, fair value less than 12 months	140,321	425,983
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses	(2,058)	(29,692)
Total of investment securities, fair value, 12 Months or More	1,039,330	1,020,418
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses	(250,121)	(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	147,273	167,139
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(94,900)	(98,548)
Obligations of states and political subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	166
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(1)
Estimated fair value, 12 months or more	683	1,228
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(7)	(20)
Held to maturity, Estimated fair value, Less than 12 months	1,026	3,084
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(5)	(4)
Held to maturity, Estimated fair value, 12 months or more	3,558	1,430
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(22)	(24)
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	12,107	38,492
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(65)	(190)
Estimated fair value, 12 months or more	8,804	6,017
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(164)	(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	121,487	297,133
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(692)	(14,188)
Estimated fair value, 12 months or more	773,409	751,077
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(124,955)	(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, Less than 12 months
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses
Estimated fair value, 12 months or more	919	15,029
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(26)	(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		2,871
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses		(335)
Estimated fair value, 12 months or more	6,043	4,863
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(1,022)	(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	95,685	73,635
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(26,900)	(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	5,535	9,883
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(1,295)	(4,500)
Estimated fair value, 12 months or more	2,956
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	$ (2,125)