Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 144,050	$ 117,804	$ 110,410
Securities available for sale:
Unrealized gains (losses) arising during the period, net of tax expense (benefit) of $13,593, $(10,121) and $2,760	25,242	(18,796)	5,125
Reclassification adjustment for gains included in net income, net of tax expense of $(4,101), $(269) and $(216)	(7,616)	(500)	(400)
Derivative instruments:
Unrealized gains (losses) arising during the period, net of tax expense (benefit) of $3,469, $(3,454) and $(92)	6,443	(6,415)	(171)
Pension and postretirement benefit obligations:
Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(7,080), $8,083 and $(3,031)	(13,148)	15,011	(5,630)
Other comprehensive income (loss)	10,921	(10,700)	(1,076)
Comprehensive income	$ 154,971	$ 107,104	$ 109,334