Investment Securities - Summary of available-for-sale securities in unrealized loss position (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	$ 281,051	$ 328,787
Unrealized Loss, Less than 12 Months	1,418	3,309
Fair Value, 12 Months or More	96,663	1,555,326
Unrealized Loss, 12 Months or More	1,590	34,941
Fair Value, Total	377,714	1,884,113
Unrealized Loss, Total	3,008	38,250
U.S. Treasuries
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months		0
Unrealized Loss, Less than 12 Months		0
Fair Value, 12 Months or More		120,391
Unrealized Loss, 12 Months or More		2,172
Fair Value, Total		120,391
Unrealized Loss, Total		2,172
US Government agencies
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	404	0
Unrealized Loss, Less than 12 Months	1	0
Fair Value, 12 Months or More	0	21,519
Unrealized Loss, 12 Months or More	0	275
Fair Value, Total	404	21,519
Unrealized Loss, Total	1	275
State and political subdivisions
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months		15,160
Unrealized Loss, Less than 12 Months		28
Fair Value, 12 Months or More		133,500
Unrealized Loss, 12 Months or More		1,688
Fair Value, Total		148,660
Unrealized Loss, Total		1,716
Residential mortgage-backed securities, Agency
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	228,611	80,202
Unrealized Loss, Less than 12 Months	576	332
Fair Value, 12 Months or More	18,294	723,094
Unrealized Loss, 12 Months or More	150	19,792
Fair Value, Total	246,905	803,296
Unrealized Loss, Total	726	20,124
Residential mortgage-backed securities, Non-agency
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months		154,381
Unrealized Loss, Less than 12 Months		476
Fair Value, 12 Months or More		52,266
Unrealized Loss, 12 Months or More		1,298
Fair Value, Total		206,647
Unrealized Loss, Total		1,774
Commercial mortgage-backed, Agency
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	0	0
Unrealized Loss, Less than 12 Months	0	0
Fair Value, 12 Months or More	33,517	355,292
Unrealized Loss, 12 Months or More	128	7,339
Fair Value, Total	33,517	355,292
Unrealized Loss, Total	128	7,339
Commercial mortgage-backed, Non-agency
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	0	4,552
Unrealized Loss, Less than 12 Months	0	594
Fair Value, 12 Months or More	4,864	0
Unrealized Loss, 12 Months or More	263	0
Fair Value, Total	4,864	4,552
Unrealized Loss, Total	263	594
Corporate bonds
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	19,742	0
Unrealized Loss, Less than 12 Months	216	0
Fair Value, 12 Months or More	998	117,296
Unrealized Loss, 12 Months or More	2	1,921
Fair Value, Total	20,740	117,296
Unrealized Loss, Total	218	1,921
Asset-backed securities
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	32,294	74,492
Unrealized Loss, Less than 12 Months	625	1,879
Fair Value, 12 Months or More	38,990	31,968
Unrealized Loss, 12 Months or More	1,047	456
Fair Value, Total	71,284	106,460
Unrealized Loss, Total	$ 1,672	$ 2,335