Operating Revenue by Geographic Location (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating revenue by geographic location      
Revenue $ 552,091 $ 588,117 $ 589,009
Australia-Asia      
Operating revenue by geographic location      
Revenue 389,098 413,662 403,697
Europe      
Operating revenue by geographic location      
Revenue $ 162,993 $ 174,455 $ 185,312