Securities - HTM in an Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Number of Securities
Unrealized loss for less than twelve months | item	89	40
Unrealized loss for twelve months or longer | item	65	63
Total | item	154	103
Fair Value
Unrealized loss for less than twelve months	$ 145,793	$ 93,489
Unrealized loss for twelve months or longer	186,008	191,404
Total	331,801	284,893
Unrealized Losses
Unrealized loss for less than twelve months	2,889	656
Unrealized loss for twelve months or longer	9,063	5,995
Total	$ 11,952	$ 6,651
Bonds
Number of Securities
Unrealized loss for less than twelve months | item	1	1
Unrealized loss for twelve months or longer | item	3	3
Total | item	4	4
Fair Value
Unrealized loss for less than twelve months	$ 5,743	$ 5,950
Unrealized loss for twelve months or longer	31,449	31,877
Total	37,192	37,827
Unrealized Losses
Unrealized loss for less than twelve months	257	50
Unrealized loss for twelve months or longer	1,566	1,138
Total	$ 1,823	$ 1,188
Residential mortgage-backed Securities
Number of Securities
Unrealized loss for less than twelve months | item	4
Total | item	4
Fair Value
Unrealized loss for less than twelve months	$ 24,822
Total	24,822
Unrealized Losses
Unrealized loss for less than twelve months	328
Total	$ 328
Commercial mortgage-backed securities
Number of Securities
Unrealized loss for less than twelve months | item	13	7
Unrealized loss for twelve months or longer | item	2	2
Total | item	15	9
Fair Value
Unrealized loss for less than twelve months	$ 54,085	$ 39,396
Unrealized loss for twelve months or longer	12,349	12,659
Total	66,434	52,055
Unrealized Losses
Unrealized loss for less than twelve months	1,211	271
Unrealized loss for twelve months or longer	692	464
Total	$ 1,903	$ 735
Collateralized mortgage obligations
Number of Securities
Unrealized loss for less than twelve months | item	11	10
Unrealized loss for twelve months or longer | item	14	12
Total | item	25	22
Fair Value
Unrealized loss for less than twelve months	$ 35,120	$ 37,064
Unrealized loss for twelve months or longer	124,446	128,270
Total	159,566	165,334
Unrealized Losses
Unrealized loss for less than twelve months	654	264
Unrealized loss for twelve months or longer	5,293	3,705
Total	$ 5,947	$ 3,969
States and political subdivisions
Number of Securities
Unrealized loss for less than twelve months | item	60	22
Unrealized loss for twelve months or longer | item	46	46
Total | item	106	68
Fair Value
Unrealized loss for less than twelve months	$ 26,023	$ 11,079
Unrealized loss for twelve months or longer	17,764	18,598
Total	43,787	29,677
Unrealized Losses
Unrealized loss for less than twelve months	439	71
Unrealized loss for twelve months or longer	1,512	688
Total	$ 1,951	$ 759