Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 18,594	$ 15,738	$ 45,505	$ 43,867
Other comprehensive income (loss), net of tax
Net change in fair value of securities available for sale	1,211	(850)	544	3,462
Net change in fair value of cash flow hedges	132	578	596	2,085
Net change in other comprehensive income for defined benefit postretirement plans	110	(39)	309	(117)
Total other comprehensive income (loss)	1,453	(311)	1,449	5,430
Total comprehensive income	$ 20,047	$ 15,427	$ 46,954	$ 49,297