Investment Securities - Summary of held-to-maturity securities in unrealized loss position (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Summary of held to maturity securities in an unrealized loss position
Fair Value, Less than 12 Months	$ 13,641	$ 104,322
Unrealized Loss, Less than 12 Months	107	1,626
Fair Value, 12 Months or More	174,812	65,868
Unrealized Loss, 12 Months or More	7,274	1,977
Fair Value, Total	188,453	170,190
Unrealized Loss, Total	7,381	3,603
State and political subdivisions
Summary of held to maturity securities in an unrealized loss position
Fair Value, Less than 12 Months	7,062	8,969
Unrealized Loss, Less than 12 Months	46	178
Fair Value, 12 Months or More	34,146	0
Unrealized Loss, 12 Months or More	2,145	0
Fair Value, Total	41,208	8,969
Unrealized Loss, Total	2,191	178
Residential mortgage-backed securities
Summary of held to maturity securities in an unrealized loss position
Fair Value, Less than 12 Months	6,579	82,893
Unrealized Loss, Less than 12 Months	61	1,377
Fair Value, 12 Months or More	136,376	64,337
Unrealized Loss, 12 Months or More	5,033	1,912
Fair Value, Total	142,955	147,230
Unrealized Loss, Total	5,094	3,289
Commercial mortgage-backed securities
Summary of held to maturity securities in an unrealized loss position
Fair Value, Less than 12 Months	0	12,460
Unrealized Loss, Less than 12 Months	0	71
Fair Value, 12 Months or More	4,290	1,531
Unrealized Loss, 12 Months or More	96	65
Fair Value, Total	4,290	13,991
Unrealized Loss, Total	$ 96	$ 136