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Financial instruments - Schedule of Outstanding Interest Rate Derivatives (Detail)
12 Months Ended
Dec. 31, 2016
USD ($)
5.6400% [Member]  
Derivative [Line Items]  
Fixed per annum rate swapped for LIBOR 5.64%
Derivative, notional amount $ 645,970,000
Effective date Aug. 31, 2007
Ending date Aug. 31, 2017
5.6400% [Member] | Maximum [Member]  
Derivative [Line Items]  
Derivative, notional amount $ 645,970,000
5.4200% [Member]  
Derivative [Line Items]  
Fixed per annum rate swapped for LIBOR 5.42%
Derivative, notional amount $ 416,053,000
Effective date Sep. 06, 2007
Ending date May 31, 2024
5.4200% [Member] | Maximum [Member]  
Derivative [Line Items]  
Derivative, notional amount $ 416,053,000
5.6000% [Member]  
Derivative [Line Items]  
Fixed per annum rate swapped for LIBOR 5.60%
Derivative, notional amount $ 148,800,000
Effective date Jun. 23, 2010
Ending date Dec. 23, 2021
5.6000% [Member] | Maximum [Member]  
Derivative [Line Items]  
Derivative, notional amount $ 148,800,000
5.9450% [Member]  
Derivative [Line Items]  
Fixed per annum rate swapped for LIBOR 5.945%
Derivative, notional amount $ 131,544,000
Effective date Jan. 30, 2014
Ending date May 31, 2019
5.9450% [Member] | Maximum [Member]  
Derivative [Line Items]  
Derivative, notional amount $ 131,544,000
5.2600% [Member]  
Derivative [Line Items]  
Fixed per annum rate swapped for LIBOR 5.26%
Derivative, notional amount $ 87,100,000
Effective date Jul. 03, 2006
Ending date Feb. 26, 2021
5.2600% [Member] | Maximum [Member]  
Derivative [Line Items]  
Derivative, notional amount $ 87,100,000
5.8700% [Member]  
Derivative [Line Items]  
Fixed per annum rate swapped for LIBOR 5.87%
Derivative, notional amount $ 0
Effective date Aug. 31, 2017
Ending date Nov. 28, 2025
5.8700% [Member] | Maximum [Member]  
Derivative [Line Items]  
Derivative, notional amount $ 620,390,000