Investment Securities - Gross unrealized losses (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Investment securities available-for-sale
Unrealized losses less than 12 months	$ (34,576)	$ (10,290)
Fair value less than 12 months	570,503	578,270
Unrealized losses over 12 months	(23,764)	(1,681)
Fair value over 12 months	219,169	55,084
Total Unrealized losses	(58,340)	(11,971)
Total Fair value	789,672	633,354
Held-to-maturity investment securities
Unrealized losses less than 12 months	(22,325)	(3,494)
Fair value less than 12 months	315,324	257,583
Total Unrealized losses	(22,325)	(3,494)
Total Fair value	315,324	257,583
Unrealized losses less than 12 months	(56,901)	(13,784)
Fair value less than 12 months	885,827	835,853
Unrealized losses over 12 months	(23,764)	(1,681)
Fair value over 12 months	219,169	55,084
Total Unrealized losses	(80,665)	(15,465)
Total Fair value	1,104,996	890,937
Obligations of state and political agencies.
Held-to-maturity investment securities
Unrealized losses less than 12 months	(7,594)	(349)
Fair value less than 12 months	131,278	53,210
Total Unrealized losses	(7,594)	(349)
Total Fair value	131,278	53,210
Mortgage backed securities - Residential agency
Investment securities available-for-sale
Unrealized losses less than 12 months	(30,707)	(10,156)
Fair value less than 12 months	462,087	554,811
Unrealized losses over 12 months	(23,764)	(1,681)
Fair value over 12 months	219,167	55,082
Total Unrealized losses	(54,471)	(11,837)
Total Fair value	681,254	609,893
Held-to-maturity investment securities
Unrealized losses less than 12 months	(14,731)	(3,145)
Fair value less than 12 months	184,046	204,373
Unrealized losses over 12 months	(14,731)
Total Unrealized losses		(3,145)
Total Fair value	184,046	204,373
Mortgage backed securities - Commercial
Investment securities available-for-sale
Unrealized losses less than 12 months	(2,576)	(123)
Fair value less than 12 months	70,176	17,470
Total Unrealized losses	(2,576)	(123)
Total Fair value	70,176	17,470
Asset backed securities
Investment securities available-for-sale
Fair value over 12 months	2	2
Total Fair value	2	2
Corporate bonds
Investment securities available-for-sale
Unrealized losses less than 12 months	(1,293)	(11)
Fair value less than 12 months	38,240	5,989
Total Unrealized losses	(1,293)	(11)
Total Fair value	$ 38,240	$ 5,989