Derivative Financial Instruments (Company's Derivative Instruments of Fuel Hedge Agreements) (Details) - Fuel [Member] - Fuel Hedge Agreement One [Member]	6 Months Ended
	Jun. 30, 2018 gal / M $ / gal
Derivative [Line Items]
Date entered	2016-07
Notional amount (in gallons per month) | gal / M	1,000,000
Diesel rate paid fixed (per gallon) | $ / gal	2.6345
Diesel rate received variable	DOE Diesel Fuel Index*	[1]
Effective date	2018-01
Expiration date	2018-12

[1] If the national U.S. on-highway average price for a gallon of diesel fuel ("average price"), as published by the U.S. Department of Energy ("DOE"), exceeds the contract price per gallon, the Company receives the difference between the average price and the contract price (multiplied by the notional number of gallons) from the counterparty. If the average price is less than the contract price per gallon, the Company pays the difference to the counterparty.