Stockholders' equity - Stock Options and Restricted Stock Units (Details) - 2006 Stock Incentive Plan - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Number of Shares
Outstanding, beginning of period (in shares)	1,300,000
Stock options granted (in shares)	300,000
Exercised (in shares)	(300,000)
Forfeited (in shares)	(100,000)
Outstanding, end of period (in shares)	1,200,000
Exercisable, end of period (in shares)	600,000
Weighted-Average Exercise Price
Outstanding, beginning of period (in dollars per share)	$ 49,070,000.00
Stock options grant price (in dollars per share)	89,470,000
Exercised (in dollars per share)	35,910,000
Forfeited (in dollars per share)	71,770,000
Outstanding, end of period (in dollars per share)	60,830,000
Exercisable, end of period (in dollars per share)	$ 47,280,000
Aggregate Intrinsic Value
Outstanding	$ 3.0	$ 53.4
Exercisable	$ 3.0
Performance Stock Units (PSUs) and Restricted Stock Units (RSUs)
Number of Shares
Outstanding, beginning of period (in shares)	1,100,000
Restricted stock units granted (in shares)	700,000
Vested (in shares)	(500,000)
Forfeited (in shares)	(200,000)
Outstanding, end of period (in shares)	1,100,000
Weighted-Average Grant Price
Outstanding, beginning of period (in dollars per share)	$ 63,300,000
Restricted stock units grant price (in dollars per share)	76,720,000
Vested (in dollars per share)	61,760,000
Forfeited (in dollars per share)	74,420,000
Outstanding, end of period (in dollars per share)	$ 70,820,000
Aggregate Intrinsic Value
Outstanding	$ 47.6	$ 96.3