Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income		$ 121,622	$ 87,204	$ 76,648
Other comprehensive income (loss), net of tax
Net change in fair value of securities available for sale		(4,501)	(677)	(1,133)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax		4,829	443	2,170
Net change in other comprehensive income for defined benefit postretirement plans	[1]	1,558	(260)	78
Total other comprehensive income (loss)		1,886	(494)	1,115
Total comprehensive income		$ 123,508	$ 86,710	$ 77,763

[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.