Pension and Other Post-Retirement Benefits - Multiemployer Benefit Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Multiemployer Plans
Contributions		$ 8.4	$ 7.1	$ 8.2
Multiemployer Plan Contributions		0.00%
American Maritime Officers Pension Plan [Member]
Multiemployer Plans
Contributions	[1]	$ 2.3	1.2	1.4
Collective Bargaining		Feb. 07, 2021
Other Multiemployer Post Retirement Plans [Member]
Multiemployer Plans
Contributions		$ 6.1	$ 5.9	$ 6.8

[1] _______(1) Our contributions represented more than 5% of the total contributions to the plan during each year and no surcharge was imposed for any year. The actuary for the American Maritime Officers Pension Plan certified that the plan is in endangered status (i.e. “yellow zone” as defined by the Pension Protection Act of 2006) for the plan year beginning October 1, 2013, because of funding or liquidity problems, or both. A rehabilitation plan, as defined by the Employee Retirement Security Act of 1974, was instituted under which certain adjustable benefits were reduced or eliminated, and we are required to contribute at a negotiated rate per day worked by each employee.