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Long-Term Debt - Summary of Long-Term Debt (Details)
6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
credit_facility
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]      
Proceeds from financing related to sales and leaseback of vessels (note 8) $ 63,720,000 $ 0  
Total principal 597,925,000   $ 741,992,000
Less: unamortized discount and debt issuance costs (4,699,000)   (6,586,000)
Total debt 593,226,000   735,406,000
Less: current portion (101,264,000)   (106,236,000)
Non-current portion of long-term debt 491,962,000   629,170,000
Term Loans      
Debt Instrument [Line Items]      
Total principal 273,996,000   323,995,000
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Total principal $ 323,929,000   417,997,000
Revolving Credit Facilities Due through 2022 [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Number Of Debt Instruments | credit_facility 2    
Line of Credit Facility, Remaining Borrowing Capacity $ 61,400,000   11,800,000
Line of Credit Facility, Maximum Borrowing Capacity 385,400,000.0   $ 429,800,000.0
Long-term Debt, Maturities, Revolving Credit Facility, Decrease in Amount Available Remaining Fiscal Year 6,100,000    
Long-term Debt, Maturities, Revolving Credit Facility, Decrease in Amount Available Year Two 12,100,000    
Long-term Debt, Maturities, Revolving Credit Facility, Decrease in Amount Available Year Three 304,800,000    
Long-term Debt, Maturities, Revolving Credit Facility, Decrease in Amount Available Year Four $ 62,400,000    
Maximum [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 3.50%    
Minimum [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.30%    
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Term Loans      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate     2.00%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 2.75%   2.75%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Term Loans      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.30%   0.30%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 2.00%   2.00%