Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income		$ 31,118	$ 20,563	$ 58,673	$ 41,288
Other comprehensive income (loss), net of tax
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax		(1,924)	792	(7,392)	1,323
Net change in fair value of cash flow hedges		(112)	(190)	103	(101)
Net change in other comprehensive income for defined benefit postretirement plans	[1]	117	78	234	156
Total other comprehensive income (loss)		(1,919)	680	(7,055)	1,378
Total comprehensive income		$ 29,199	$ 21,243	$ 51,618	$ 42,666

[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.