Summary of Significant Accounting Policies (Recently Adopted and Issued Accounting Pronoucements) (Details 1) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Item Effected [Line Items]
Income from Operations	$ 67,116	$ 44,571	$ 68,361	$ 29,054	$ 109,489	$ 68,033	$ 74,140	$ 51,872	$ 209,102	$ 303,534	$ (80,825)
Income before Income Taxes	$ 54,860	$ 38,244	$ 65,899	$ 23,786	$ 64,118	$ 40,926	$ 84,999	$ 60,615	182,789	250,658	$ (120,890)
As reported [Member]
Item Effected [Line Items]
Income from Operations									209,102	303,534
Non-operating pension and postretirement income									0	0
Income before Income Taxes									182,789	250,658
Adjustment [Member]
Item Effected [Line Items]
Income from Operations									(72,699)	(80,665)
Non-operating pension and postretirement income									72,699	80,665
Income before Income Taxes									0	0
Upon Adoption] [Member]
Item Effected [Line Items]
Income from Operations									136,403	222,869
Non-operating pension and postretirement income									72,699	80,665
Income before Income Taxes									$ 182,789	$ 250,658