Share and Unit-based Plans - Directors' Phantom Stock Plan (Details)	12 Months Ended
	Dec. 31, 2019 $ / shares shares	Dec. 31, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares
Phantom stock units
Share and unit-based plans
Number of common shares into which units can be converted (shares)	1
Units
Balance at beginning of year (shares)	0	4,054	5,845
Granted (shares)	23,690	10,380	8,760
Vested (shares)	(16,474)	(12,193)	(10,551)
Forfeited (shares)	0	(2,241)	0
Balance at end of year (shares)	7,216	0	4,054
Weighted Average Grant Date Fair Value (in dollars per share)
Balance at beginning of year (in dollars per share) | $ / shares	$ 0	$ 79.82	$ 81.47
Granted (in dollars per share) | $ / shares	40.26	49.55	68.93
Vested (in dollars per share) | $ / shares	38.94	54.40	71.69
Forfeited (in dollars per share) | $ / shares	0	77.91	0
Balance at end of year (in dollars per share) | $ / shares	$ 43.29	$ 0	$ 79.82
Director's Phantom Stock Plan
Share and unit-based plans
Deferral period for grant of units (in years)	3 years
Number of common shares into which units can be converted (shares)	1
Maximum shares authorized under plan (shares)	500,000
Shares available for issuance under plan (in shares)	134,638