Compensation plans - Activity for equity compensation awards (Details) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Restricted Stock Awards
Restricted Stock and Performance Shares Awards
Outstanding at the beginning of the period (in shares) | shares	362
Granted (in shares) | shares	340
Forfeited (in shares) | shares	(47)
Vested (in shares) | shares	(183)
Outstanding at the end of the period (in shares) | shares	472
Weighted-average grant-date fair value (per share)
Outstanding at the beginning of the period (USD per share) | $ / shares	$ 52.90
Granted (in USD per share) | $ / shares	54.68
Forfeited (USD per share) | $ / shares	57.39
Vested (USD per share) | $ / shares	44.86
Outstanding at the end of the period (USD per share) | $ / shares	$ 56.87
Weighted-average exercise price (per share)
Intrinsic value of vested restricted stock awards | $	$ 9.3
Stock Option Awards
Stock Option Awards
Outstanding at the beginning of the period (in shares) | shares	3
Expired or canceled (in shares) | shares	(1)
Outstanding at the end of the period (in shares) | shares	2
Weighted-average exercise price (per share)
Outstanding at the end of the period (in USD per share) | $ / shares	$ 235.08
Expired or canceled (in USD per share) | $ / shares	346.80
Outstanding at end of the period (in USD per share) | $ / shares	$ 136.83
Performance Share Awards
Restricted Stock and Performance Shares Awards
Outstanding at the beginning of the period (in shares) | shares	48
Outstanding at the end of the period (in shares) | shares	48
Weighted-average grant-date fair value (per share)
Outstanding at the beginning of the period (USD per share) | $ / shares	$ 89.76
Outstanding at the end of the period (USD per share) | $ / shares	$ 101.48