Share and Unit-based Plans - Stock Awards and Stock Units Roll Forward Activity (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares shares	Dec. 31, 2016 $ / shares shares	Dec. 31, 2015 $ / shares shares
Stock awards
Shares or Units
Balance at beginning of year (shares) | shares	0	1,612	9,189
Granted (shares) | shares	0	0	0
Vested (shares) | shares	0	(1,612)	(7,577)
Balance at end of year (shares) | shares	0	0	1,612
Weighted Average Grant Date Fair Value
Balance at beginning of year (in dollars per share) | $ / shares	$ 0.00	$ 62.01	$ 59.25
Granted (in dollars per share) | $ / shares	0.00	0.00	0.00
Vested (in dollars per share) | $ / shares	0.00	62.01	58.67
Balance at end of year (in dollars per share) | $ / shares	$ 0.00	$ 0.00	$ 62.01
Stock units
Shares or Units
Balance at beginning of year (shares) | shares	148,428	132,086	144,374
Granted (shares) | shares	86,827	85,601	77,282
Vested (shares) | shares	(81,205)	(69,259)	(86,761)
Forfeited (shares) | shares	(2,695)	0	(2,809)
Balance at end of year (shares) | shares	151,355	148,428	132,086
Weighted Average Grant Date Fair Value
Balance at beginning of year (in dollars per share) | $ / shares	$ 78.53	$ 74.58	$ 59.94
Granted (in dollars per share) | $ / shares	66.46	79.22	86.53
Vested (in dollars per share) | $ / shares	75.62	71.82	61.29
Forfeited (in dollars per share) | $ / shares	69.57	0.00	86.72
Balance at end of year (in dollars per share) | $ / shares	$ 73.32	$ 78.53	$ 74.58
Number of common shares into which units can be converted (shares)	1