Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	$ 97,591	$ 139,295
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(452)	(2,053)
Estimated fair value, 12 months or more	811,238	888,499
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(120,272)	(155,199)
Held to maturity, Estimated fair value, Less than 12 months	1,065	1,026
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(7)	(5)
Held to maturity, Estimated fair value, 12 months or more	156,844	150,831
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(81,462)	(94,922)
Total investment securities, fair value less than 12 months	98,656	140,321
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses	(459)	(2,058)
Total of investment securities, fair value, 12 Months or More	968,082	1,039,330
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses	(201,734)	(250,121)
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	155,050	147,273
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(81,438)	(94,900)
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	572	166
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(3)	(1)
Estimated fair value, 12 months or more	680	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	1,065	1,026
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses	(7)	(5)
Held to maturity, Estimated fair value, 12 months or more	1,794	3,558
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses	(24)	(22)
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	7,966	12,107
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(41)	(65)
Estimated fair value, 12 months or more	7,450	8,804
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(137)	(164)
Mortgage-backed Securities [Member] | Privately Issued Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Estimated fair value, Less than 12 months	89,046	121,487
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(401)	(692)
Estimated fair value, 12 months or more	695,871	773,409
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(97,588)	(124,955)
Mortgage-backed Securities [Member] | Privately Issued Commercial Mortgage-Backed Securities [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
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses		(26)
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	6,039	6,043
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(1,038)	(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
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses
Estimated fair value, 12 months or more	101,198	95,685
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses	(21,505)	(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	7	5,535
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses	(7)	(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)