Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 159,649	$ 144,050	$ 117,804
Securities available for sale:
Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(1,561), $13,593, and $(10,121)	(2,899)	25,242	(18,796)
Reclassification adjustment for gains included in net income, net of tax expense of $288, $4,101 and $269	(534)	(7,616)	(500)
Derivative instruments:
Unrealized gains (losses) arising during the period, net of tax expense (benefit) of $1,989, $4,629 and $(3,165)	3,694	8,597	(5,878)
Reclassification adjustment for gains included in net income, net of tax expense of $1,137, $1,160 and $289	(2,111)	(2,154)	(537)
Pension and postretirement benefit obligations:
Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(1,766), $(7,080) and $8,083	(3,280)	(13,148)	15,011
Other comprehensive (loss) income	(5,130)	10,921	(10,700)
Comprehensive income	$ 154,519	$ 154,971	$ 107,104