Organization, Basis of Presentation and Recent Accounting Pronouncements (Recent Accounting Pronouncements)(Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Item Effected [Line Items]
Income from Operations	$ 68,361	$ 74,140	$ 97,415	$ 126,012
Income before income taxes	65,899	84,999	89,685	145,614
As reported [Member]
Item Effected [Line Items]
Income from Operations	68,361	74,140	97,415	126,012	$ 303,534
Non-operating pension and postretirement benefit income	0	0	0	0	0
Income before income taxes	65,899	84,999	89,685	145,614	250,658
Adjustment [Member]
Item Effected [Line Items]
Income from Operations	(18,620)	(15,584)	(37,421)	(31,261)	(80,665)
Non-operating pension and postretirement benefit income	18,620	15,584	37,421	31,261	80,665
Income before income taxes	0	0	0	0	0
Upon Adoption [Member]
Item Effected [Line Items]
Income from Operations	49,741	58,556	59,994	94,751	222,869
Non-operating pension and postretirement benefit income	18,620	15,584	37,421	31,261	80,665
Income before income taxes	$ 65,899	$ 84,999	$ 89,685	$ 145,614	$ 250,658