Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrealized losses		$ 112	$ (661)	$ 683	$ 729
Tax benefit (expense)		(3,661)	(2,192)	(10,367)	(7,642)
Total reclassifications for the period, net of tax		(132)	(174)	44	(2,689)
Unrealized gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income		(70)	(80)	31	(2,629)
Realized net loss on interest rate swap derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income		499	406	1,809	864
Net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income	[1]	(596)	(547)	(1,789)	(1,642)
Prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income	[1]	1	1	5	5
Amortization of pension plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassified from accumulated other comprehensive income		(595)	(546)	(1,784)	(1,637)
Tax benefit		125	115	375	343
Total reclassifications for the period, net of tax		(470)	(431)	(1,409)	(1,294)
Unrealized losses on securities transferred from held to maturity to available for sale: | Reclassification out of accumulated other comprehensive income | Unrealized gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of unrealized losses	[2]	(112)	661	(683)	(729)
Tax benefit (expense)		23	(139)	143	153
Net of tax		(89)	522	(540)	(576)
Unrealized gains and losses on available for sale securities | Reclassification out of accumulated other comprehensive income | Unrealized gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)		(9)	156	(150)	399
Net of tax		33	(585)	564	(1,501)
Realized net (loss) gain on sale of securities	[3]	42	(741)	714	(1,900)
Interest rate swap derivatives | Reclassification out of accumulated other comprehensive income | Realized net loss on interest rate swap derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)		(100)	(81)	(366)	(168)
Net of tax		377	303	1,378	631
Realized net gain (loss) on interest rate swap derivatives	[4]	477	384	1,744	799
Interest rate swap derivatives | Reclassification out of accumulated other comprehensive income | Amortization of unrealized gains on terminated interest rate swap derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit (expense)		(5)	(5)	(14)	(14)
Net of tax		17	17	51	51
Realized net gain (loss) on interest rate swap derivatives	[4]	$ 22	$ 22	$ 65	$ 65

[1] These accumulated other comprehensive income (“AOCI”) components are included in the computation of net periodic pension cost (income) presented in “Note 9 - Employee Benefit Plans.”

[2] Included in interest income on the consolidated statements of income.

[3] Listed as net gain (loss) on sale of securities available for sale on the consolidated statements of income.

[4] Included in interest expense for Federal Home Loan Bank of Dallas (“FHLB”) borrowings on the consolidated statements of income.