Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income		$ 33,015	$ 23,852	$ 91,688	$ 65,140
Other comprehensive income (loss), net of tax
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax		(2,262)	188	(9,654)	1,511
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax		(405)	109	(302)	8
Net change in other comprehensive income for defined benefit postretirement plans	[1]	117	78	351	234
Total other comprehensive income (loss)		(2,550)	375	(9,605)	1,753
Total comprehensive income		$ 30,465	$ 24,227	$ 82,083	$ 66,893

[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, 2017, filed with the Securities and Exchange Commission.