Other Non-current Assets (Details) (USD $)
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 16, 2014
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Other Non-current Assets        
Fair value of swaps $ 664,000 $ 2,472,000    
Other assets 1,952,000 883,000    
Total 68,506,000 29,120,000    
Interest from Fair value unwinding 600,000      
Unearned revenue 30,412,000      
Revenue 552,091,000 588,117,000 589,009,000  
Current portion of unearned revenue 13,719,000 7,305,000    
ZIM        
Other Non-current Assets        
Receivable   25,765,000    
Equity participation ZIM 28,693,000      
Number of vessels whose charterparties were revised 6     6
Percent of outstanding shares after the restructuring, in exchange for charter rate reductions and cancellation   7.40%    
Equity investment in ZIM       28,700,000
Unearned revenue 30,400,000     39,100,000
Current portion of unearned revenue 6,000,000      
ZIM | Unsecured interest bearing notes maturing in 2023        
Other Non-current Assets        
Principal amount of unsecured notes received       49,900,000
ZIM | Unsecured 3% Series 1 ZIM Notes due 2023        
Other Non-current Assets        
Receivable 6,274,000      
Principal amount of unsecured notes received       8,800,000
Interest rate (as a percent)       3.00%
Fair value of notes       6,100,000
Interest from Fair value unwinding 6,300,000      
ZIM | Unsecured 5% Series 2 ZIM Notes due 2023        
Other Non-current Assets        
Receivable 30,923,000      
Principal amount of unsecured notes received       41,100,000
Interest rate (as a percent)       5.00%
Interest payable quarterly in cash (as a percent)       3.00%
Interest payable in kind (as a percent)       2.00%
Fair value of notes       30,100,000
Interest from Fair value unwinding 30,900,000      
Noncash Interest income $ 400,000