Securities - Amortized cost and fair value of held to maturity and available for sale securities (Details 5) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost	$ 1,938,238		$ 1,817,774		$ 1,684,240
Securities available-for-sale	1,937,264		1,834,593		1,701,583
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost	214,947		244,184		282,750
Held-To-Maturity, Fair value	226,695		261,131		299,971
State and political subdivisions
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost, Within 1 year	1,830
Available for Sale, Fair Value, Within 1 year	1,868
Available for Sale, Amortized Cost, 1 to 5 years	14,572
Available for Sale, Fair Value, 1 to 5 years	15,460
Available for Sale, Amortized Cost, 5 to 10 years	5,119
Available for Sale, Fair Value, 5 to 10 years	5,207
Available for Sale, Amortized Cost, More than 10 years	848
Available for Sale, Fair Value, More than 10 years	905
Available for Sale, Amortized Cost	22,369
Securities available-for-sale	23,440
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost, Within 1 year
Held to Maturity, Fair Value, Within 1 year
Held to Maturity, Amortized Cost, 1 to 5 years	10,860
Held to Maturity, Fair Value, 1 to 5 years	11,724
Held to Maturity, Amortized Cost, 5 to 10 years	25,594
Held to Maturity, Fair Value, 5 to 10 years	27,728
Held to Maturity, Amortized Cost, More than 10 years	15,303
Held to Maturity, Fair Value, More than 10 years	16,633
Held to Maturity, Amortized Cost	51,757		51,780		51,801
Held-To-Maturity, Fair value	56,085		57,266		57,387
Corporate bonds
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost, 1 to 5 years	38,183
Available for Sale, Fair Value, 1 to 5 years	38,451
Available for Sale, Amortized Cost, 5 to 10 years	211,517
Available for Sale, Fair Value, 5 to 10 years	205,234
Available for Sale, Amortized Cost, More than 10 years	10,764
Available for Sale, Fair Value, More than 10 years	10,183
Available for Sale, Amortized Cost	260,464
Securities available-for-sale	253,868
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost, 1 to 5 years
Held to Maturity, Fair Value, 1 to 5 years
Held to Maturity, Amortized Cost, 5 to 10 years
Held to Maturity, Fair Value, 5 to 10 years
Held to Maturity, Amortized Cost, More than 10 years
Held to Maturity, Fair Value, More than 10 years
Held to Maturity, Amortized Cost
Held-To-Maturity, Fair value
Asset-backed securities
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost, 1 to 5 years	40,537
Available for Sale, Fair Value, 1 to 5 years	40,644
Available for Sale, Amortized Cost, 5 to 10 years	153,281
Available for Sale, Fair Value, 5 to 10 years	155,038
Available for Sale, Amortized Cost, More than 10 years	48,872
Available for Sale, Fair Value, More than 10 years	49,162
Available for Sale, Amortized Cost	242,690
Securities available-for-sale	244,844
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost, 1 to 5 years
Held to Maturity, Fair Value, 1 to 5 years
Held to Maturity, Amortized Cost, 5 to 10 years
Held to Maturity, Fair Value, 5 to 10 years
Held to Maturity, Amortized Cost, More than 10 years
Held to Maturity, Fair Value, More than 10 years
Held to Maturity, Amortized Cost
Held-To-Maturity, Fair value
Other
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost, More than 10 years	2,526
Available for Sale, Fair Value, More than 10 years	2,526
Available for Sale, Amortized Cost	2,526
Securities available-for-sale	2,526
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost, More than 10 years
Held to Maturity, Fair Value, More than 10 years
Held to Maturity, Amortized Cost
Held-To-Maturity, Fair value
Total securities other than mortgage-backed securities
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost, Within 1 year	1,830
Available for Sale, Fair Value, Within 1 year	1,868
Available for Sale, Amortized Cost, 1 to 5 years	93,292
Available for Sale, Fair Value, 1 to 5 years	94,555
Available for Sale, Amortized Cost, 5 to 10 years	369,917
Available for Sale, Fair Value, 5 to 10 years	365,479
Available for Sale, Amortized Cost, More than 10 years	63,010
Available for Sale, Fair Value, More than 10 years	62,776
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost, Within 1 year
Held to Maturity, Fair Value, Within 1 year
Held to Maturity, Amortized Cost, 1 to 5 years	10,860
Held to Maturity, Fair Value, 1 to 5 years	11,724
Held to Maturity, Amortized Cost, 5 to 10 years	25,594
Held to Maturity, Fair Value, 5 to 10 years	27,728
Held to Maturity, Amortized Cost, More than 10 years	15,303
Held to Maturity, Fair Value, More than 10 years	16,633
Mortgage-backed securities
Available for Sale - Amortized Cost and Fair Value
Available for Sale, Amortized Cost	1,410,189
Securities available-for-sale	1,412,586
Held to Maturity - Amortized Cost and Fair Value
Held to Maturity, Amortized Cost	163,190	[1]	192,404	[1]	230,949	[1]
Held-To-Maturity, Fair value	$ 170,610	[1]	$ 203,865	[1]	$ 242,584	[1]

[1] All are residential type mortgage-backed securities