Organization, Basis of Presentation and Recent Accounting Pronouncements (Recent Accounting Pronouncements)(Details 2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Item Effected [Line Items]
Income from Operations	$ 44,571	$ 68,033	$ 141,986	$ 194,045
Income before income taxes	38,244	40,926	127,929	186,540
As reported [Member]
Item Effected [Line Items]
Income from Operations	44,571	68,033	141,986	194,045	$ 303,534
Non-operating pension and postretirement benefit income	0	0	0	0	0
Income before income taxes	38,244	40,926	127,929	186,540	250,658
Adjustment [Member]
Item Effected [Line Items]
Income from Operations	(17,621)	(15,705)	(55,042)	(46,966)	(80,665)
Non-operating pension and postretirement benefit income	17,621	15,705	55,042	46,966	80,665
Income before income taxes	0	0	0	0	0
Upon Adoption [Member]
Item Effected [Line Items]
Income from Operations	26,950	52,328	86,944	147,079	222,869
Non-operating pension and postretirement benefit income	17,621	15,705	55,042	46,966	80,665
Income before income taxes	$ 38,244	$ 40,926	$ 127,929	$ 186,540	$ 250,658