Equity - Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity
Dividends declared per common share (in dollars per share)	$ 3.40	$ 3.20	$ 3.00
Aggregated dividend paid	$ 174.5	$ 163.7	$ 153.0
Minimum percentage of adjusted funds from operations as cash dividend (as a percent)	50.00%
Percentage of REIT taxable income as cash dividend (as a percent)	100.00%