Employee benefit plans - Weighted-Average Asset Allocations (Details)	Dec. 31, 2019	Dec. 31, 2018
United States
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	100.00%	100.00%
Canada
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	100.00%	100.00%
U.K.
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	100.00%	100.00%
Retirement Plan | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	0.00%	48.30%
Retirement Plan | United States | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	91.60%	38.40%
Retirement Plan | United States | Alternative
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	5.70%	13.30%
Retirement Plan | United States | Cash and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	2.70%	0.00%
Retirement Plan | Canada | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	67.90%	55.40%
Retirement Plan | Canada | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	31.60%	44.00%
Retirement Plan | Canada | Alternative
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	0.00%	0.00%
Retirement Plan | Canada | Cash and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	0.50%	0.60%
Retirement Plan | U.K. | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	42.10%	38.90%
Retirement Plan | U.K. | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	57.30%	60.50%
Retirement Plan | U.K. | Alternative
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	0.00%	0.00%
Retirement Plan | U.K. | Cash and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	0.60%	0.60%
Retiree Health and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	100.00%	100.00%
Retiree Health and Life Insurance Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	0.00%	48.30%
Retiree Health and Life Insurance Plans | Debt securities
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	91.60%	38.40%
Retiree Health and Life Insurance Plans | Alternative
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	5.70%	13.30%
Retiree Health and Life Insurance Plans | Cash and short-term investments
Defined Benefit Plan Disclosure [Line Items]
Total (percent)	2.70%	0.00%