Equity compensation plans (Details) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 22, 2015 $ / shares	Jan. 31, 2015	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)
Units
Outstanding at the beginning of the period (in shares) | shares					2,884,574
Granted (in shares) | shares					2,483,237
Vested and redeemed (in shares) | shares					(1,216,252)
Outstanding at the end of the period (in shares) | shares			4,151,559		4,151,559
Weighted-Average Fair Value per Unit
Outstanding at the beginning of the period (in dollars per share)					$ 2.22
Granted (in dollars per share)					2.02
Vested and redeemed (in dollars per share)					2.24
Outstanding at the end of the period (in dollars per share)			$ 2.09		$ 2.09
Cash payments made for vested notional units | $					$ 0.8	$ 0.7
Long-term incentive plan
Weighted-Average Fair Value per Unit
Compensation expense related to LTIP | $			$ 1.0	$ 0.9	$ 1.5	$ 1.7
Transition equity participation agreement
Transition equity participation agreement
Number of transition notional shares outstanding | shares					539,904
Vesting percentage		50.00%
Vesting period					4 years
Period after which remaining portions vests					2 years
Share Price	$ 3.18
Minimum market price required for 3 consecutive months to vest remaining shares	$ 4.77
Transition equity participation agreement | Minimum
Transition equity participation agreement
Number of consecutive months, weighted average Canadian dollar price must exceed market price of share by 50% for vesting					3 months
Minimum percentage by which weighted average Canadian dollar price exceeds market price	50.00%