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	$ 199,204	$ 170,891	$ 159,649
Securities available for sale:
Unrealized gains arising during the period, net of tax benefit of $3,224, $7,477 and $1,561	(5,744)	(13,886)	(2,899)
Reclassification adjustment for gains included in net income, net of tax expense of $230, $249 and $288	(411)	(463)	(534)
Derivative instruments:
Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(426), $2,739 and $1,887	(759)	5,086	3,504
Reclassification adjustment for gains included in net income, net of tax expense of $26, $685 and $1,035	(46)	(1,272)	(1,921)
Pension and postretirement benefit obligations:
Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(1), $161 and $(1,766)	(7)	299	(3,280)
Other comprehensive income (loss)	(6,967)	(10,236)	(5,130)
Comprehensive income	$ 192,237	$ 160,655	$ 154,519