Shareholders' equity - Shareholders' rights plan (Details)	12 Months Ended
	Dec. 31, 2018 multiplier $ / shares shares
Class of Stock [Line Items]
Fraction of newly issued share of Series A junior participating preferred stock that could be purchased for each Right | shares	0.01
Purchase price of preferred stock (in dollars per share)	$ 70
Percentage of assets or earning power sold allowing the right to receive common stock of acquiring company	50.00%
Value of a share of the Company's common stock to be received upon the exercise of the right if the Company is acquired or 50 percent or more of its consolidated assets or earning power is sold, expressed as a multiple of the exercise price of the right | multiplier	2
Percentage of common stock to be acquired in which the Company's board of directors may exchange all or some of the rights, minimum	15.00%
Percentage of common stock to be acquired in which the Company's board of directors may exchange all or some of the rights, maximum (less than)	50.00%
Redemption price per right at any time prior to the tenth day following the date on which a person acquires beneficial ownership of 15% or more of the outstanding class A or class B common shares (in dollars per share)	$ 0.05
Class A common shares
Class of Stock [Line Items]
Minimum percentage of common stock to be acquired for shareholder rights to become exercisable	15.00%
Minimum percentage of common stock to be acquired in a tender offer or exchange offer for shareholder rights to become exercisable	30.00%
Class B common shares
Class of Stock [Line Items]
Minimum percentage of common stock to be acquired for shareholder rights to become exercisable	15.00%
Minimum percentage of common stock to be acquired in a tender offer or exchange offer for shareholder rights to become exercisable	30.00%