Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 15,597	$ 15,535	$ 14,853	$ 13,597	$ 11,260	$ 6,637	$ 8,019	$ 7,571	$ 59,582	$ 33,487	$ 26,637
Securities available for sale:
Unrealized holding gains (losses) on securities									7,731	(6,869)	3,305
Non-credit related portion of other-than-temporary impairment on securities									0	0	0
Reclassification adjustment for gains realized in net income									(232)	71	(1,170)
Amortization of unrealized holding gains on securities transferred to the held to maturity category									(156)	(215)	(350)
Total securities available for sale									7,343	(7,013)	1,785
Derivative instruments:
Unrealized holding (losses) gains on derivative instruments									(1,622)	1,325	(1,336)
Reclassification adjustment for gains realized in net income									0	(125)	(311)
Totals derivative instruments									(1,622)	1,200	(1,647)
Defined benefit pension and post-retirement benefit plans:
Net (loss) gain arising during the period									(1,529)	1,957	(756)
Less amortization of net actuarial loss recognized in net periodic pension cost									183	384	264
Total defined benefit pension and post-retirement benefit plans									(1,346)	2,341	(492)
Other comprehensive (loss) income, net of tax									4,375	(3,472)	(354)
Comprehensive income									$ 63,957	$ 30,015	$ 26,283