Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 20,563	$ 20,374	$ 41,288	$ 38,985
Other comprehensive income, net of tax
Net change in fair value of securities available for sale	792	1,854	1,323	5,935
Net change in fair value of cash flow hedges	(190)	(144)	(101)	(21)
Net change in other comprehensive income for defined benefit postretirement plans	78	61	156	121
Total other comprehensive income	680	1,771	1,378	6,035
Total comprehensive income	$ 21,243	$ 22,145	$ 42,666	$ 45,020