Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Sep. 29, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 07, 2018
Class of Stock [Line Items]
Shares issued under dividend reinvestment plan (in shares)		17,952	17,911	15,619
Issuance of preferred shares, net of costs		$ 0	$ 144,991	$ 0
Preferred shares, par value			$ 0.01
Net cash proceeds of common stock		$ 130,918	$ 118,583	$ 329,103
At The Market Equity Program
Class of Stock [Line Items]
Shares issued		987,383	826,517
Net cash proceeds of common stock		$ 126,100	$ 108,300
Aggregate offering price of common shares					$ 400,000
Weighted average price per common share		$ 129.19	$ 132.56
Remaining capacity to issue		$ 272,400
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Commissions related to sales of common shares		1,300	$ 1,100
At The Market Equity Program | Other Offering Costs
Class of Stock [Line Items]
Commissions related to sales of common shares		$ 200	$ 200
5.0% Series C Cumulative Redeemable Preferred Shares
Class of Stock [Line Items]
Preferred shares, shares outstanding		6,000	6,000
Preferred shares, percentage	5.00%	5.00%	5.00%	5.00%
Preferred shares, liquidation preference per share	$ 25,000	$ 25,000	$ 25,000
Issuance of preferred shares, net of costs	$ 145,000
Preferred shares, par value	$ 0.01
5.417% Series 1 Cumulative Convertible Preferred Shares
Class of Stock [Line Items]
Preferred shares, shares outstanding		399,896	399,896	399,896
Preferred shares, percentage		5.417%	5.417%	5.417%
Preferred shares, liquidation preference per share		$ 25	$ 25	$ 25
Preferred shares, par value		0.01	$ 0.01	$ 0.01
Conversion price per share		$ 104.69
Depositary Shares | 5.0% Series C Cumulative Redeemable Preferred Shares
Class of Stock [Line Items]
Preferred shares, liquidation preference per share	$ 25.00
Common shares
Class of Stock [Line Items]
Shares issued under dividend reinvestment plan (in shares)		17,952	17,911	15,619
Shares issued		987,461	826,592	2,156,671
Preferred Shares
Class of Stock [Line Items]
Shares issued			6,000
Preferred Shares | Depositary Shares | 5.0% Series C Cumulative Redeemable Preferred Shares
Class of Stock [Line Items]
Shares issued	6,000,000