Fair Value Measurements and Derivative Instruments (Derivative Instruments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative instruments disclosure
Aggregate cost of ships on order	$ 8,400.0
Amount deposited for cost of ships on order	$ 316.1
Percentage of aggregate cost exposed to fluctuations in the euro exchange rate	66.70%	58.20%
Exchange gains (losses) recorded in other income (expense)	$ 39.8	$ 34.6	$ 49.5
Foreign currency forward | Not Designated
Derivative instruments disclosure
Notional amount	642.4
Change in fair value of foreign currency forward contracts recognized in earnings	51.1	(55.5)	(48.6)
Exchange gains (losses) recorded in other income (expense)	39.8	34.6	$ 49.5
Foreign exchange contracts
Derivative instruments disclosure
Notional amount	$ 1,300.0	$ 2,400.0