v2.4.0.8
Long-Term Debt - Additional Information (Detail) (Revolving Credit Facilities [Member], USD $)
In Millions, unless otherwise specified
 6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Debt Instrument [Line Items]    
Number of debt instruments 3  
Maximum borrowings available $ 775.8  
Undrawn amount of revolving credit facility 217.8  
Amount reduced under revolving credit facilities, 2013 47.0  
Amount reduced under revolving credit facilities, 2014 94.0  
Amount reduced under revolving credit facilities, 2015 120.9  
Amount reduced under revolving credit facilities, 2016 89.1  
Amount reduced under revolving credit facilities, 2017 395.9  
Amount reduced under revolving credit facilities, thereafter 28.9  
Number of first priority mortgages granted on vessels as debt collateral 21  
Minimum hull coverage ratio 105.00%  
Actual hull coverage ratio 114.00%  
Amount of liquidity and undrawn revolving credit line 35.0  
Liquidity and undrawn revolving credit line as percentage of debt 5.00%  
Maintain the greater of free cash liquidity for revolving credit facility $ 100.0  
Minimum percentage to maintaining revolving credit facility 7.50%  
Minimum [Member]
    
Debt Instrument [Line Items]    
Percentage of margin 0.45% 0.45%
Maximum [Member]
    
Debt Instrument [Line Items]    
Percentage of margin 0.60% 0.60%