Investment Securities, Available-for-Sale (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value
Fair Value, Less than 12 Months	$ 956,214	$ 1,001,588
Fair Value, 12 Months or More	126,944	23,558
Fair Value, Total	1,083,158	1,025,146
Unrealized Loss
Unrealized Loss, Less than 12 Months	(5,254)	(8,204)
Unrealized Loss, 12 Months or More	(1,762)	(1,103)
Unrealized Loss, Total	(7,016)	(9,307)
State and local governments [Member]
Fair Value
Fair Value, Less than 12 Months	98,691	26,434
Fair Value, 12 Months or More	5,418	9,948
Fair Value, Total	104,109	36,382
Unrealized Loss
Unrealized Loss, Less than 12 Months	(823)	(90)
Unrealized Loss, 12 Months or More	(85)	(488)
Unrealized Loss, Total	(908)	(578)
Corporate bonds [Member]
Fair Value
Fair Value, Less than 12 Months	13,024	31,782
Fair Value, 12 Months or More	0
Fair Value, Total	13,024	31,782
Unrealized Loss
Unrealized Loss, Less than 12 Months	(35)	(1,264)
Unrealized Loss, 12 Months or More	0
Unrealized Loss, Total	(35)	(1,264)
Collateralized debt obligations [Member]
Fair Value
Fair Value, 12 Months or More	2,762	5,366
Fair Value, Total	2,762	5,366
Unrealized Loss
Unrealized Loss, 12 Months or More	(85)	(282)
Unrealized Loss, Total	(85)	(282)
Residential mortgage-backed securities [Member]
Fair Value
Fair Value, Less than 12 Months	844,499	943,372
Fair Value, 12 Months or More	118,764	8,244
Fair Value, Total	963,263	951,616
Unrealized Loss
Unrealized Loss, Less than 12 Months	(4,396)	(6,850)
Unrealized Loss, 12 Months or More	(1,592)	(333)
Unrealized Loss, Total	$ (5,988)	$ (7,183)