Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income		$ 87,204	$ 76,648	$ 64,960
Other comprehensive income (loss), net of tax
Net change in fair value of securities available for sale		(677)	(1,133)	(2,083)
Net change in fair value of cash flow hedges		443	2,170	1,199
Net change in other comprehensive income for defined benefit postretirement plans	[1]	(260)	78	564
Total other comprehensive income (loss)		(494)	1,115	(320)
Total comprehensive income		$ 86,710	$ 77,763	$ 64,640

[1] The amortization of prior service costs is included in the computation of net periodic pension costs as disclosed in Note 15 - Employee Benefit Plans.