Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 21,160	$ 16,220	$ 15,394	$ 15,240	$ 15,597	$ 15,535	$ 14,853	$ 13,597	$ 68,014	$ 59,582	$ 33,487
Securities available for sale:
Unrealized holding (losses) gains on securities									(351)	7,731	(6,869)
Reclassification adjustment for gains realized in net income(1)									(59)	(232)	71
Amortization of unrealized holding gains on securities transferred to the held to maturity category									(110)	(156)	(215)
Total securities available for sale									(520)	7,343	(7,013)
Derivative instruments:
Unrealized holding (losses) gains on derivative instruments									(249)	(1,622)	1,325
Reclassification adjustment for gains realized in net income(2)									0	0	(125)
Total derivative instruments									(249)	(1,622)	1,200
Defined benefit pension and post-retirement benefit plans:
Net (loss) gain arising during the period									(1,435)	(1,529)	1,957
Less amortization of net actuarial loss recognized in net periodic pension cost									267	183	384
Total defined benefit pension and post-retirement benefit plans									(1,168)	(1,346)	2,341
Other comprehensive (loss) income, net of tax									(1,937)	4,375	(3,472)
Comprehensive income									$ 66,077	$ 63,957	$ 30,015