Securities - HTM in an Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Number of Securities
Unrealized loss for less than twelve months | item	78	40
Unrealized loss for twelve months or longer | item	79	63
Total | item	157	103
Fair Value
Unrealized loss for less than twelve months	$ 108,238	$ 93,489
Unrealized loss for twelve months or longer	212,437	191,404
Total	320,675	284,893
Unrealized Losses
Unrealized loss for less than twelve months	3,188	656
Unrealized loss for twelve months or longer	12,609	5,995
Total	$ 15,797	$ 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	$ 5,674	$ 5,950
Unrealized loss for twelve months or longer	31,060	31,877
Total	36,734	37,827
Unrealized Losses
Unrealized loss for less than twelve months	326	50
Unrealized loss for twelve months or longer	1,957	1,138
Total	$ 2,283	$ 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	$ 22,199
Total	22,199
Unrealized Losses
Unrealized loss for less than twelve months	605
Total	$ 605
Commercial mortgage-backed securities
Number of Securities
Unrealized loss for less than twelve months | item	11	7
Unrealized loss for twelve months or longer | item	7	2
Total | item	18	9
Fair Value
Unrealized loss for less than twelve months	$ 40,122	$ 39,396
Unrealized loss for twelve months or longer	37,325	12,659
Total	77,447	52,055
Unrealized Losses
Unrealized loss for less than twelve months	1,057	271
Unrealized loss for twelve months or longer	2,008	464
Total	$ 3,065	$ 735
Collateralized mortgage obligations
Number of Securities
Unrealized loss for less than twelve months | item	7	10
Unrealized loss for twelve months or longer | item	18	12
Total | item	25	22
Fair Value
Unrealized loss for less than twelve months	$ 16,262	$ 37,064
Unrealized loss for twelve months or longer	124,873	128,270
Total	141,135	165,334
Unrealized Losses
Unrealized loss for less than twelve months	519	264
Unrealized loss for twelve months or longer	6,745	3,705
Total	$ 7,264	$ 3,969
States and political subdivisions
Number of Securities
Unrealized loss for less than twelve months | item	55	22
Unrealized loss for twelve months or longer | item	51	46
Total | item	106	68
Fair Value
Unrealized loss for less than twelve months	$ 23,981	$ 11,079
Unrealized loss for twelve months or longer	19,179	18,598
Total	43,160	29,677
Unrealized Losses
Unrealized loss for less than twelve months	681	71
Unrealized loss for twelve months or longer	1,899	688
Total	$ 2,580	$ 759