Investment Securities - Summary of available-for-sale securities in unrealized loss position (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	$ 328,787	$ 1,233,399
Unrealized Loss, Less than 12 Months	3,309	10,440
Fair Value, 12 Months or More	1,555,326	341,662
Unrealized Loss, 12 Months or More	34,941	9,406
Fair Value, Total	1,884,113	1,575,061
Unrealized Loss, Total	38,250	19,846
U.S. Treasuries
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	0	121,113
Unrealized Loss, Less than 12 Months	0	912
Fair Value, 12 Months or More	120,391	0
Unrealized Loss, 12 Months or More	2,172	0
Fair Value, Total	120,391	121,113
Unrealized Loss, Total	2,172	912
US Government agencies
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	0	1,976
Unrealized Loss, Less than 12 Months	0	13
Fair Value, 12 Months or More	21,519	1,677
Unrealized Loss, 12 Months or More	275	13
Fair Value, Total	21,519	3,653
Unrealized Loss, Total	275	26
State and political subdivisions
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	15,160	61,494
Unrealized Loss, Less than 12 Months	28	365
Fair Value, 12 Months or More	133,500	5,131
Unrealized Loss, 12 Months or More	1,688	31
Fair Value, Total	148,660	66,625
Unrealized Loss, Total	1,716	396
Residential mortgage-backed securities
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	234,583	655,180
Unrealized Loss, Less than 12 Months	808	5,692
Fair Value, 12 Months or More	775,360	242,501
Unrealized Loss, 12 Months or More	21,090	7,004
Fair Value, Total	1,009,943	897,681
Unrealized Loss, Total	21,898	12,696
Commercial mortgage-backed securities
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	4,552	309,025
Unrealized Loss, Less than 12 Months	594	3,007
Fair Value, 12 Months or More	355,292	86,422
Unrealized Loss, 12 Months or More	7,339	2,231
Fair Value, Total	359,844	395,447
Unrealized Loss, Total	7,933	5,238
Corporate bonds
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	0	55,916
Unrealized Loss, Less than 12 Months	0	325
Fair Value, 12 Months or More	117,296	900
Unrealized Loss, 12 Months or More	1,921	100
Fair Value, Total	117,296	56,816
Unrealized Loss, Total	1,921	425
Asset-backed securities
Summary of available for sale securities in an unrealized loss position
Fair Value, Less than 12 Months	74,492	28,695
Unrealized Loss, Less than 12 Months	1,879	126
Fair Value, 12 Months or More	31,968	5,031
Unrealized Loss, 12 Months or More	456	27
Fair Value, Total	106,460	33,726
Unrealized Loss, Total	$ 2,335	$ 153