Share-based compensation plans - Stock Activity - RSU, PSU, DSU (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Restricted Stock Units
Shares (in thousands)
Nonvested, beginning balance, shares	320,200
Granted, shares	119,100
Forfeited or expired, shares	(5,900)
Vested, shares	(108,000.0)
Nonvested, ending balance, shares	325,400	320,200
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning balance (dollars per share)	$ 65.89
Granted (dollars per share)	80.77
Forfeited or expired (dollars per share)	67.99
Vested (dollars per share)	68.86
Nonvested, ending balance (dollars per share)	$ 70.31	$ 65.89
Performance Shares Units
Shares (in thousands)
Nonvested, beginning balance, shares	698,500
Granted, shares	207,000.0
Forfeited or expired, shares	(8,600)
Vested, shares	(228,800)
Nonvested, ending balance, shares	668,100	698,500
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning balance (dollars per share)	$ 72.15
Granted (dollars per share)	81.34
Forfeited or expired (dollars per share)	67.43
Vested (dollars per share)	80.47
Nonvested, ending balance (dollars per share)	$ 72.21	$ 72.15
Actual shares earned and distributed (shares)		437,900
Target shares (shares)		228,800
Deferred Stock Units
Shares (in thousands)
Nonvested, beginning balance, shares	19,200
Granted, shares	0
Vested, shares	0
Nonvested, ending balance, shares	19,200	19,200
Weighted-Average Grant Date Fair Value Per Share
Nonvested, beginning balance (dollars per share)	$ 62.43
Granted (dollars per share)	0
Vested (dollars per share)	0
Nonvested, ending balance (dollars per share)	$ 62.43	$ 62.43