Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Net securities losses	$ 412	$ (4)	$ 575	$ (2)
Interest income	77,527	71,930	226,754	210,042
Other noninterest income	3,048	3,945	11,341	10,069
Other noninterest expense	4,577	4,483	13,324	13,880
Interest expense	10,677	6,917	26,977	19,135
Income tax (expense) benefit	8,578	11,342	23,699	30,321
Net income	29,807	22,876	83,914	64,514
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Total reclassifications during the period, net of tax	(138)	334	(96)	2,004
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Income tax (expense) benefit	(42)	(83)	(133)	(760)
Net income	126	133	400	1,229
Losses on AFS Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Net securities losses	0	4	0	2
Amortization of Unrealized Gains Related to Securities Transfer [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Interest income	168	212	533	675
Impairment Write-Down of Equity Security [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Other noninterest income			0	1,312
Net Unrealized (Gains) on Cash Flow hedges Reclassified to Interest (Income) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Interest expense	(638)	(155)	(1,537)	(96)
Income tax (expense) benefit	159	59	384	37
Net income	(479)	(96)	(1,153)	(59)
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Income tax (expense) benefit	(71)	(183)	(219)	(516)
Net income	215	297	657	834
Amortization of Net Losses [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Other noninterest expense	263	481	809	1,351
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss) [Abstract]
Other noninterest expense	$ 23	$ (1)	$ 67	$ (1)