Share and Unit-based Plans - SARs Narrative (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jan. 06, 2016	Dec. 31, 2015	Dec. 08, 2015	Dec. 07, 2015	Dec. 31, 2014
SARs
Share and unit-based plans
Term of award (in years)	10 years
Grant date of award (in dollars per share)	$ 9.67
Volatility rate (as a percent)	25.85%
Dividend yield (as a percent)	3.69%
Risk free rate (as a percent)	1.20%
Current value (in dollars per share)	$ 59.57
Expected term (in years)	8 years
Stock appreciation units balance (shares)	235,439	284,146	427,968	417,783	417,783	407,823	772,639
Stock appreciation weighted average price (in dollars per share)	$ 53.83	$ 53.85	$ 53.85	$ 55.13	$ 55.13	$ 56.49	$ 56.67
SARS granted prior to 2012
Share and unit-based plans
Grant date of award (in dollars per share)	$ 7.68
Volatility rate (as a percent)	22.52%
Dividend yield (as a percent)	5.23%
Risk free rate (as a percent)	3.15%
Current value (in dollars per share)	$ 61.17
Expected term (in years)	8 years