Investment Securities - Summary of available for sale securities in an unrealized loss position (Details 4) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	$ 493,835	$ 433,595	$ 450,250
Unrealized Loss, Less than 12 Months	4,068	2,994	3,536
Fair Value, 12 Months or More	77,017	93,913	90,308
Unrealized Loss, 12 Months or More	3,941	7,039	11,715
Fair Value, Total	570,852	527,508	540,558
Unrealized Loss, Total	8,009	10,033	15,251
U.S. Government agencies
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months			9,905
Unrealized Loss, Less than 12 Months			90
Fair Value, 12 Months or More
Unrealized Loss, 12 Months or More
Fair Value, Total			9,905
Unrealized Loss, Total			90
State and political subdivisions
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	1,185	3,674
Unrealized Loss, Less than 12 Months	2	17
Fair Value, 12 Months or More	10	10	11
Unrealized Loss, 12 Months or More	2	2	3
Fair Value, Total	1,195	3,684	11
Unrealized Loss, Total	4	19	3
Mortgage-backed securities
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	399,263	326,485	405,039
Unrealized Loss, Less than 12 Months	3,544	2,092	574
Fair Value, 12 Months or More			21,067
Unrealized Loss, 12 Months or More			16
Fair Value, Total	399,263	326,485	426,106
Unrealized Loss, Total	3,544	2,092	590
Corporate bonds
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	21,323	21,248	35,306
Unrealized Loss, Less than 12 Months	145	136	2,872
Fair Value, 12 Months or More	77,007	93,903	69,230
Unrealized Loss, 12 Months or More	3,939	7,037	11,696
Fair Value, Total	98,330	115,151	104,536
Unrealized Loss, Total	4,084	7,173	14,568
Asset-backed securities
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	72,064	82,188
Unrealized Loss, Less than 12 Months	377	749
Fair Value, 12 Months or More
Unrealized Loss, 12 Months or More
Fair Value, Total	72,064	82,188
Unrealized Loss, Total	$ 377	$ 749