Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income		$ 27,555	$ 20,725
Other comprehensive income (loss), net of tax
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax		(5,468)	531
Net change in fair value of cash flow hedges		215	89
Net change in other comprehensive income for defined benefit postretirement plans	[1]	117	78
Total other comprehensive income (loss)		(5,136)	698
Total comprehensive income		$ 22,419	$ 21,423

[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.