Long-Term Debt - Financial Covenants in Credit Agreements (Details)		12 Months Ended
	Dec. 18, 2015 USD ($)	Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
Total leverage ratio, actual		2.93
Debt service coverage ratio, actual		3.73
Minimum liquidity balance, actual		$ 151,900,000
Total leverage ratio, covenant requirement		3.75
Debt service coverage ratio, covenant requirement		2.00
Minimum liquidity balance	$ 25,000,000	$ 25,000,000