Share and Unit-based Plans - Directors' Phantom Stock Plan (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 $ / 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)	5,845	0	9,269
Granted (shares)	8,760	21,088	13,351
Vested (shares)	(10,551)	(15,243)	(20,162)
Forfeited (shares)	0	0	(2,458)
Balance at end of year (shares)	4,054	5,845	0
Weighted Average Grant Date Fair Value (in dollars per share)
Balance at beginning of year (in dollars per share) | $ / shares	$ 81.47	$ 0.00	$ 58.35
Granted (in dollars per share) | $ / shares	68.93	80.21	78.72
Vested (in dollars per share) | $ / shares	71.69	79.73	72.17
Forfeited (in dollars per share) | $ / shares	0.00	0.00	55.62
Balance at end of year (in dollars per share) | $ / shares	$ 79.82	$ 81.47	$ 0.00
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)	169,755