Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	$ 185,779	$ 139,295
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(2,005)	(2,053)
Estimated fair value, 12 months or more	115,290	888,499
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(21,252)	(155,199)
Held to maturity, Estimated fair value, Less than 12 months	1,274,104	1,026
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(29,085)	(5)
Held to maturity, Estimated fair value, 12 months or more	169,434	150,831
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(56,909)	(94,922)
Total investment securities, fair value less than 12 months	1,459,883	140,321
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses	(31,090)	(2,058)
Total of investment securities, fair value, 12 Months or More	284,724	1,039,330
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses	(78,161)	(250,121)
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	174,503	12,107
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(1,126)	(65)
Estimated fair value, 12 months or more	5,731	8,804
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(128)	(164)
Held to maturity, Estimated fair value, Less than 12 months	1,259,353
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(29,015)
Held to maturity, Estimated fair value, 12 months or more
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses
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
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses
Held to maturity, Estimated fair value, 12 months or more	167,876	147,273
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(56,891)	(94,900)
Privately Issued Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months		121,487
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses		(692)
Estimated fair value, 12 months or more	107	774,328
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(5)	(124,981)
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	577	166
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(2)	(1)
Estimated fair value, 12 months or more	559	683
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(4)	(7)
Held to maturity, Estimated fair value, Less than 12 months	14,751	1,026
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(70)	(5)
Held to maturity, Estimated fair value, 12 months or more	1,558	3,558
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(18)	(22)
U.S. Treasury and federal agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	6,346
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(25)
Estimated fair value, 12 months or more
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses
Collateralized Debt Obligations [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	5,995	6,043
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(1,107)	(1,022)
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	1,576
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(7)
Estimated fair value, 12 months or more	102,898	95,685
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(20,008)	(26,900)
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	2,777	5,535
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(845)	(1,295)
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)