Summary of Significant Accounting Policies (Recently Adopted and Issued Accounting Pronoucements) (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Item Effected [Line Items]
Operating expenses	$ 432,706	$ 448,920	$ 440,655	$ 365,151	$ 371,922	$ 374,987	$ 381,747	$ 325,687	$ 1,687,432	$ 1,454,343	$ 1,270,030
Selling, general and administrative expenses	152,624	131,081	141,378	225,045	225,477	228,051	208,973	225,289	650,128	887,790	896,097
Income from Operations	75,582	60,739	65,626	44,214	49,459	26,950	49,741	10,253	246,161	136,403	222,869
Non-operating pension and postretirement benefit income, net	53,900	22,214	23,041	21,386	17,657	17,621	18,620	18,801	120,541	72,699	80,665
Income Before Income Taxes	$ 69,138	$ 135,070	$ 62,735	$ 56,565	$ 54,860	$ 38,244	$ 65,899	$ 23,786	$ 323,508	182,789	250,658
Accounting Standards Update 2017-07 [Member]
Item Effected [Line Items]
Operating expenses										1,454,343	1,270,030
Selling, general and administrative expenses										887,790	896,097
Income from Operations										136,403	222,869
Non-operating pension and postretirement benefit income, net										72,699	80,665
Income Before Income Taxes										182,789	250,658
Accounting Standards Update 2017-07 [Member] | Previously Reported [Member]
Item Effected [Line Items]
Operating expenses										1,359,842	1,180,945
Selling, general and administrative expenses										909,592	904,517
Income from Operations										209,102	303,534
Non-operating pension and postretirement benefit income, net										0	0
Income Before Income Taxes										182,789	250,658
Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
Item Effected [Line Items]
Operating expenses										94,501	89,085
Selling, general and administrative expenses										(21,802)	(8,420)
Income from Operations										(72,699)	(80,665)
Non-operating pension and postretirement benefit income, net										72,699	80,665
Income Before Income Taxes										$ 0	$ 0