Organization, Basis of Presentation and Recent Accounting Pronouncements (Recent Accounting Pronouncements)(Details 2) - USD ($) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Item Effected [Line Items]
Operating expenses	$ 365,151				$ 325,687
Selling, general and administrative	225,045				225,289
Income from Operations	44,214	$ 49,459	$ 26,950	$ 49,741	10,253	$ 136,403	$ 222,869
Income before income taxes	$ 56,565	$ 54,860	$ 38,244	$ 65,899	23,786	$ 182,789	$ 250,658
Accounting Standards Update 2017-07 [Member]
Item Effected [Line Items]
Operating expenses					325,687
Selling, general and administrative					225,289
Income from Operations					10,253
Non-operating pension and postretirement benefit income					18,801
Income before income taxes					23,786
As previously reported [Member] | Accounting Standards Update 2017-07 [Member]
Item Effected [Line Items]
Operating expenses					300,666
Selling, general and administrative					231,509
Income from Operations					29,054
Non-operating pension and postretirement benefit income					0
Income before income taxes					23,786
Adjustment [Member] | Accounting Standards Update 2017-07 [Member]
Item Effected [Line Items]
Operating expenses					25,021
Selling, general and administrative					(6,220)
Income from Operations					(18,801)
Non-operating pension and postretirement benefit income					18,801
Income before income taxes					$ 0