Securities - HTM in an Unrealized Loss Position (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Number of Securities
Unrealized loss for less than twelve months | item	51	101
Unrealized loss for twelve months or longer | item	50	2
Total | item	101	103
Fair Value
Unrealized loss for less than twelve months	$ 131,170	$ 282,825
Unrealized loss for twelve months or longer	118,240	1,863
Total	249,410	284,688
Unrealized Losses
Unrealized loss for less than twelve months	1,384	7,043
Unrealized loss for twelve months or longer	3,596	13
Total	$ 4,980	$ 7,056
Bonds
Number of Securities
Unrealized loss for less than twelve months | item		4
Unrealized loss for twelve months or longer | item	3
Total | item	3	4
Fair Value
Unrealized loss for less than twelve months		$ 33,225
Unrealized loss for twelve months or longer	$ 32,124
Total	32,124	33,225
Unrealized Losses
Unrealized loss for less than twelve months		1,287
Unrealized loss for twelve months or longer	891
Total	$ 891	$ 1,287
Residential mortgage-backed Securities
Number of Securities
Unrealized loss for less than twelve months | item		2
Total | item		2
Fair Value
Unrealized loss for less than twelve months		$ 13,178
Total		13,178
Unrealized Losses
Unrealized loss for less than twelve months		6
Total		$ 6
Commercial mortgage-backed securities
Number of Securities
Unrealized loss for less than twelve months | item	3	5
Unrealized loss for twelve months or longer | item	2	1
Total | item	5	6
Fair Value
Unrealized loss for less than twelve months	$ 13,805	$ 18,891
Unrealized loss for twelve months or longer	12,823	1,401
Total	26,628	20,292
Unrealized Losses
Unrealized loss for less than twelve months	130	588
Unrealized loss for twelve months or longer	380	5
Total	$ 510	$ 593
Collateralized mortgage obligations
Number of Securities
Unrealized loss for less than twelve months | item	12	19
Unrealized loss for twelve months or longer | item	5
Total | item	17	19
Fair Value
Unrealized loss for less than twelve months	$ 100,054	$ 187,669
Unrealized loss for twelve months or longer	57,570
Total	157,624	187,669
Unrealized Losses
Unrealized loss for less than twelve months	1,066	3,372
Unrealized loss for twelve months or longer	1,484
Total	$ 2,550	$ 3,372
States and political subdivisions
Number of Securities
Unrealized loss for less than twelve months | item	36	71
Unrealized loss for twelve months or longer | item	40	1
Total | item	76	72
Fair Value
Unrealized loss for less than twelve months	$ 17,311	$ 29,862
Unrealized loss for twelve months or longer	15,723	462
Total	33,034	30,324
Unrealized Losses
Unrealized loss for less than twelve months	188	1,790
Unrealized loss for twelve months or longer	841	8
Total	$ 1,029	$ 1,798