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	$ 64,960	$ 59,845	$ 50,254
Other comprehensive income (loss), net of tax
Net change in fair value of securities available for sale	(2,083)	5,412	(7,501)
Net change in fair value of cash flow hedges	1,199	2,256	3,735
Net change in other comprehensive income for defined benefit postretirement plans	564	(2,366)	858
Total other comprehensive income (loss)	(320)	5,302	(2,908)
Total comprehensive income	$ 64,640	$ 65,147	$ 47,346