v3.3.1.900
Equity compensation plans (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Jan. 22, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Long-term incentive plan        
Units        
Outstanding at the beginning of the period (in shares)   1,443,254 766,988 492,535
Granted (in shares)   1,007,726 1,776,083 597,031
Additional shares from dividends (in shares)   59,996 178,114 64,576
Forfeited (in shares)   (136,894) (294,037) (184,458)
Vested and redeemed (in shares)   (1,075,681) (983,894) (202,696)
Outstanding at the end of the period (in shares)   1,298,401 1,443,254 766,988
Weighted-Average Fair Value per Unit        
Outstanding at the beginning of the period (in dollars per share)   $ 3.28 $ 7.86 $ 13.90
Granted (in dollars per share)   2.75 2.64 4.91
Additional shares from dividends (in dollars per share)   2.87 3.79 8.74
Forfeited (in dollars per share)   3.75 6.68 8.17
Vested and redeemed (in dollars per share)   3.21 4.78 13.48
Outstanding at the end of the period (in dollars per share)   $ 2.88 $ 3.28 $ 7.86
Fair value of outstanding notional units   $ 3.7 $ 4.6 $ 4.8
Weighted average vesting term   1 year 8 months 12 days    
Unrecognized compensation expense   $ 1.7    
Cost recognition period   1 year 8 months 12 days    
Compensation expense related to LTIP   $ 3.1 3.5 2.2
Cash payments made for vested notional units   $ 0.9 $ 0.7 $ 0.9
Transition equity participation agreement        
Transition equity participation agreement        
Shares available for grant   550,869    
Vesting percentage   50.00%    
Vesting period   4 years    
Period after which remaining portions vests   2 years    
Number of consecutive months, weighted average Canadian dollar price exceeds market price of share by 50%   3 months    
Market price (in dollars per share) $ 2.58      
Minimum percentage by which weighted average Canadian dollar price exceeds market price 50.00%