Securities - Securities with Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale securities
Fair value, less than 12 months	$ 7,154	$ 111,648
Unrealized losses, less than 12 months	40	886
Fair value, 12 months or longer	1,077,153	1,109,535
Unrealized losses, 12 months or longer	18,166	31,256
Fair Value	1,084,307	1,221,183
Unrealized Losses	18,206	32,142
Held-to-maturity debt securities
Fair value, less than 12 months	4,885	13,107
Unrealized losses, less than 12 months	19	33
Fair value, 12 months or longer	35,130	125,128
Unrealized losses, 12 months or longer	149	1,189
Fair Value	40,015	138,235
Unrealized Losses	168	1,222
Obligations of U.S. Government sponsored entities
Available-for-sale securities
Fair value, less than 12 months	0	21,660
Unrealized losses, less than 12 months	0	183
Fair value, 12 months or longer	420,382	449,141
Unrealized losses, 12 months or longer	3,434	7,370
Fair Value	420,382	470,801
Unrealized Losses	3,434	7,553
Held-to-maturity debt securities
Fair value, less than 12 months	0	4,980
Unrealized losses, less than 12 months	0	9
Fair value, 12 months or longer	35,130	125,128
Unrealized losses, 12 months or longer	149	1,189
Fair Value	35,130	130,108
Unrealized Losses	149	1,198
Obligations of U.S. states and political subdivisions
Available-for-sale securities
Fair value, less than 12 months	1,582	11,971
Unrealized losses, less than 12 months	7	19
Fair value, 12 months or longer	31,020	49,756
Unrealized losses, 12 months or longer	162	914
Fair Value	32,602	61,727
Unrealized Losses	169	933
U.S. Government agencies
Available-for-sale securities
Fair value, less than 12 months	0	16,854
Unrealized losses, less than 12 months	0	22
Fair value, 12 months or longer	93,706	96,247
Unrealized losses, 12 months or longer	2,645	3,710
Fair Value	93,706	113,101
Unrealized Losses	2,645	3,732
U.S. Government sponsored entities
Available-for-sale securities
Fair value, less than 12 months	5,572	61,163
Unrealized losses, less than 12 months	33	662
Fair value, 12 months or longer	529,595	512,216
Unrealized losses, 12 months or longer	11,875	18,937
Fair Value	535,167	573,379
Unrealized Losses	11,908	19,599
U.S. corporate debt securities
Available-for-sale securities
Fair value, less than 12 months	0	0
Unrealized losses, less than 12 months	0	0
Fair value, 12 months or longer	2,450	2,175
Unrealized losses, 12 months or longer	50	325
Fair Value	2,450	2,175
Unrealized Losses	50	325
Obligations of U.S. states and political subdivisions
Held-to-maturity debt securities
Fair value, less than 12 months	4,885	8,127
Unrealized losses, less than 12 months	19	24
Fair value, 12 months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Fair Value	4,885	8,127
Unrealized Losses	$ 19	$ 24

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