Stock-based compensation - Changes in Restricted Stock Units (Details) - RSUs - $ / shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Restricted Stock Units Outstanding
Balance at beginning of period (in shares)	1,047,071,000	826,263,000
Granted (in shares)	193,760,000	132,605,000
Vested (in shares)	(159,114,000)	(132,203,000)
Forfeited (in shares)	(29,639,000)	(14,826,000)
Balance at end of period (in shares)	1,052,078,000	811,839,000
Weighted Average Grant Date Fair Value
Balance at beginning of period (in dollars per share)	$ 26.06	$ 23.76
Granted (in dollars per share)	43.06	33.92
Vested (in dollars per share)	32.63	32.52
Forfeited (in dollars per share)	27.36	34.24
Balance at end of period (in dollars per share)	$ 27.73	$ 24.84