Accumulated Other Comprehensive Income (Loss) - Reclassifications (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Realized securities gains reclassified into income									$ 2,631	$ 3,389	$ 2,670
Income tax expense	$ (15,210)	$ (7,939)	$ (7,207)	$ (7,168)	$ (7,850)	$ (7,469)	$ (6,716)	$ (5,574)	(37,524)	(27,609)	(25,665)
Subordinated debentures and term loans									7,572	7,185	6,661
Salaries and employee benefits									119,812	102,552	101,908
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									1,667	2,422	1,006
Unrealized gains (losses) on available for sale securities
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									1,710	2,203	1,736
Unrealized gains (losses) on cash flow hedges
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									(640)	(813)	(928)
Unrealized gains (losses) on defined benefit plans
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									597	1,032
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Unrealized gains (losses) on available for sale securities
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Realized securities gains reclassified into income									2,631	3,389	2,670
Income tax expense									(921)	(1,186)	(934)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Unrealized gains (losses) on cash flow hedges
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense									345	437	499
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Unrealized gains (losses) on cash flow hedges | Interest Rate Contracts
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Subordinated debentures and term loans									(985)	(1,250)	(1,427)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) | Unrealized gains (losses) on defined benefit plans
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense									(209)	(556)	(107)
Salaries and employee benefits									806	1,588	305
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax									$ 597	$ 1,032	$ 198