Securities - AFS in an Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Number of Securities
Unrealized loss for less than twelve months | item	52	68
Unrealized loss for twelve months or longer | item	33	11
Total | item	85	79
Fair Value
Unrealized loss for less than twelve months	$ 282,014	$ 267,719
Unrealized loss for twelve months or longer	138,291	30,877
Estimated Fair Value, Total	420,305	298,596
Unrealized Loss
Unrealized loss for less than twelve months	1,213	5,863
Unrealized loss for twelve months or longer	3,989	1,223
Total	$ 5,202	$ 7,086
US Treasury Securities
Number of Securities
Unrealized loss for less than twelve months | item	6	7
Unrealized loss for twelve months or longer | item	1
Total | item	7	7
Fair Value
Unrealized loss for less than twelve months	$ 17,354	$ 21,694
Unrealized loss for twelve months or longer	399
Estimated Fair Value, Total	17,753	21,694
Unrealized Loss
Unrealized loss for less than twelve months	37	44
Unrealized loss for twelve months or longer	2
Total	$ 39	$ 44
Bonds
Number of Securities
Unrealized loss for less than twelve months | item	10	1
Total | item	10	1
Fair Value
Unrealized loss for less than twelve months	$ 77,870	$ 14,908
Estimated Fair Value, Total	77,870	14,908
Unrealized Loss
Unrealized loss for less than twelve months	133	67
Total	$ 133	$ 67
Residential mortgage-backed Securities
Number of Securities
Unrealized loss for less than twelve months | item	9	12
Unrealized loss for twelve months or longer | item	8
Total | item	17	12
Fair Value
Unrealized loss for less than twelve months	$ 76,966	$ 109,398
Unrealized loss for twelve months or longer	67,899
Estimated Fair Value, Total	144,865	109,398
Unrealized Loss
Unrealized loss for less than twelve months	557	2,941
Unrealized loss for twelve months or longer	1,747
Total	$ 2,304	$ 2,941
Commercial mortgage-backed securities
Number of Securities
Unrealized loss for less than twelve months | item	1	2
Total | item	1	2
Fair Value
Unrealized loss for less than twelve months	$ 5,076	$ 7,127
Estimated Fair Value, Total	5,076	7,127
Unrealized Loss
Unrealized loss for less than twelve months	24	58
Total	$ 24	$ 58
Collateralized mortgage obligations
Number of Securities
Unrealized loss for less than twelve months | item	13	11
Unrealized loss for twelve months or longer | item	14	8
Total | item	27	19
Fair Value
Unrealized loss for less than twelve months	$ 96,857	$ 91,144
Unrealized loss for twelve months or longer	60,719	19,320
Estimated Fair Value, Total	157,576	110,464
Unrealized Loss
Unrealized loss for less than twelve months	440	2,340
Unrealized loss for twelve months or longer	2,216	866
Total	$ 2,656	$ 3,206
Corporate securities
Number of Securities
Unrealized loss for less than twelve months | item	1	3
Total | item	1	3
Fair Value
Unrealized loss for less than twelve months	$ 1,996	$ 5,899
Estimated Fair Value, Total	1,996	5,899
Unrealized Loss
Unrealized loss for less than twelve months	2	91
Total	$ 2	$ 91
States and political subdivisions
Number of Securities
Unrealized loss for less than twelve months | item	11	32
Unrealized loss for twelve months or longer | item	8	1
Total | item	19	33
Fair Value
Unrealized loss for less than twelve months	$ 5,714	$ 17,549
Unrealized loss for twelve months or longer	2,374	450
Estimated Fair Value, Total	8,088	17,999
Unrealized Loss
Unrealized loss for less than twelve months	20	322
Unrealized loss for twelve months or longer	19	14
Total	$ 39	$ 336
Equity securities
Number of Securities
Unrealized loss for less than twelve months | item	1
Unrealized loss for twelve months or longer | item	2	2
Total | item	3	2
Fair Value
Unrealized loss for less than twelve months	$ 181
Unrealized loss for twelve months or longer	6,900	$ 11,107
Estimated Fair Value, Total	7,081	11,107
Unrealized Loss
Unrealized loss for twelve months or longer	5	343
Total	$ 5	$ 343