Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 92,188	$ 90,930	$ 68,014
Securities available for sale:
Unrealized holding losses on securities	(2,218)	(6,206)	(351)
Reclassification adjustment for gains realized in net income	(91)	(727)	(59)
Unrealized holding gains on securities transferred from held to maturity to available for sale	8,108	0	0
Amortization of unrealized holding gains on securities transferred to the held to maturity category	(173)	(61)	(110)
Total securities available for sale	5,626	(6,994)	(520)
Derivative instruments:
Unrealized holding gains (losses) on derivative instruments	536	527	(249)
Total derivative instruments	536	527	(249)
Defined benefit pension and post-retirement benefit plans:
Net gain (loss) arising during the period	1,028	31	(1,435)
Amortization of net actuarial loss recognized in net periodic pension cost	249	302	267
Reclassification adjustment for net settlement gain realized in net income	0	(235)	0
Total defined benefit pension and post-retirement benefit plans	1,277	98	(1,168)
Other comprehensive income (loss), net of tax	7,439	(6,369)	(1,937)
Comprehensive income	$ 99,627	$ 84,561	$ 66,077