Securities (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 item	Dec. 31, 2014 item
Number of Securities
Unrealized loss for less than twelve months	28	17
Unrealized loss for twelve months or longer	72	117
Total	100	134
Fair Value
Unrealized loss for less than twelve months	$ 206,903	$ 54,123
Unrealized loss for twelve months or longer	145,868	503,022
Estimated Fair Value, Total	352,771	557,145
Unrealized Loss
Unrealized loss for less than twelve months	976	227
Unrealized loss for twelve months or longer	3,081	10,535
Total	4,057	10,762
U.S. Treasury securities
Number of Securities
Unrealized loss for less than twelve months	3	4
Unrealized loss for twelve months or longer		1
Total	3	5
Fair Value
Unrealized loss for less than twelve months	3,856	7,703
Unrealized loss for twelve months or longer		1,706
Estimated Fair Value, Total	3,856	9,409
Unrealized Loss
Unrealized loss for less than twelve months	6	27
Unrealized loss for twelve months or longer		6
Total	6	33
Bonds
Number of Securities
Unrealized loss for less than twelve months	11	3
Unrealized loss for twelve months or longer	3	22
Total	14	25
Fair Value
Unrealized loss for less than twelve months	192,178	34,847
Unrealized loss for twelve months or longer	44,460	373,035
Estimated Fair Value, Total	236,638	407,882
Unrealized Loss
Unrealized loss for less than twelve months	920	153
Unrealized loss for twelve months or longer	884	7,363
Total	1,804	7,516
Residential mortgage-backed securities
Number of Securities
Unrealized loss for less than twelve months	1
Unrealized loss for twelve months or longer	1	4
Total	2	4
Fair Value
Unrealized loss for less than twelve months	1,055
Unrealized loss for twelve months or longer	1,984	11,056
Estimated Fair Value, Total	3,039	11,056
Unrealized Loss
Unrealized loss for less than twelve months	2
Unrealized loss for twelve months or longer	4	137
Total	6	137
Collateralized mortgage obligations
Number of Securities
Unrealized loss for less than twelve months	3	3
Unrealized loss for twelve months or longer	7	8
Total	10	11
Fair Value
Unrealized loss for less than twelve months	3,421	7,141
Unrealized loss for twelve months or longer	56,820	61,108
Estimated Fair Value, Total	60,241	68,249
Unrealized Loss
Unrealized loss for less than twelve months	6	40
Unrealized loss for twelve months or longer	1,509	2,260
Total	1,515	2,300
Corporate debt securities
Number of Securities
Unrealized loss for twelve months or longer	1	1
Total	1	1
Fair Value
Unrealized loss for twelve months or longer	1,973	1,939
Estimated Fair Value, Total	1,973	1,939
Unrealized Loss
Unrealized loss for twelve months or longer	24	59
Total	24	59
States and political subdivisions
Number of Securities
Unrealized loss for less than twelve months	9	7
Unrealized loss for twelve months or longer	60	81
Total	69	88
Fair Value
Unrealized loss for less than twelve months	6,282	4,432
Unrealized loss for twelve months or longer	40,631	54,178
Estimated Fair Value, Total	46,913	58,610
Unrealized Loss
Unrealized loss for less than twelve months	39	7
Unrealized loss for twelve months or longer	660	710
Total	699	717
Equity securities
Number of Securities
Unrealized loss for less than twelve months	1
Total	1
Fair Value
Unrealized loss for less than twelve months	111
Estimated Fair Value, Total	111
Unrealized Loss
Unrealized loss for less than twelve months	3
Total	$ 3