Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income		$ 35,225	$ 27,555
Other comprehensive income (loss), net of tax
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax		4,729	(5,468)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax		3,285	215
Net change in other comprehensive income for defined benefit postretirement plans	[1]	40	117
Total other comprehensive income (loss)		8,054	(5,136)
Total comprehensive income		$ 43,279	$ 22,419

[1] The amortization of prior service costs is included in the computation of net periodic pension cost as disclosed in the Employee Benefit Plans footnote in the Company's Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission.