Common Shares (Activity Under At-The-Market Share Offering Program) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total net consideration	$ 445,038,000
Common shares sold		0	4,778	0
2014 ATM program [Member]
Total net consideration		$ 2,513,600
Common shares sold		28,100
Average price per common share sold		$ 90.44