Business and Summary of Significant Accounting Principles (Reconciliation of Asset Retirement Obligations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	$ 41,155	$ 35,060
Liability incurred	4,655	1,580
Revisions in estimated cash flows	(85)	3,368
Accretion expense	1,772	1,229
Liability settled	(163)	(82)
Balance at end of period	$ 47,334	$ 41,155