Equity compensation plans (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Mar. 29, 2019	Jan. 22, 2015 $ / shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Weighted-Average Fair Value per Unit
Portion of notional units settled in common stock prior to Plan modification	66.6667%
Portion of notional units settled in cash prior to Plan modification	33.3333%
Long-term incentive plan
Units
Outstanding at the beginning of the period (in shares) | shares			3,578,092	3,952,201
Granted (in shares) | shares			1,866,748	1,724,081
Vested and redeemed (in shares) | shares			(1,702,571)	(2,071,335)
Forfeitures (in shares) | shares			(31,929)	(26,855)
Outstanding at the end of the period (in shares) | shares			3,710,340	3,578,092
Weighted-Average Fair Value per Unit
Outstanding at the beginning of the period (in dollars per share)			$ 2.38	$ 2.09
Granted (in dollars per share)			2.49	2.72
Vested and redeemed (in dollars per share)			2.34	2.10
Forfeitures (in dollars per share)			2.42	2.17
Outstanding at the end of the period (in dollars per share)			$ 2.45	$ 2.38
Fair value of outstanding notional units | $			$ 9.1	$ 8.5
Weighted average vesting term			1 year 8 months 12 days
Unrecognized compensation expense | $			$ 3.0
Cash payments made for vested notional units | $			3.3	2.1
Compensation expense related to LTIP | $			$ 4.1	$ 4.9
Transition equity participation agreement
Transition equity participation agreement
Number of transition notional shares outstanding | shares			269,952
Minimum market price required for 3 consecutive months to vest remaining shares		$ 4.77
Minimum percentage by which weighted average Canadian dollar price exceeds market price		50.00%
Share Price		$ 3.18
Notional shares vest age			62 years
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