Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Sep. 29, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 07, 2018
Class of Stock [Line Items]
Shares issued under dividend reinvestment plan (in shares)		15,909	17,952	17,911
Preferred shares, percentage		5.00%	5.00%	5.00%
Preferred shares, par value		$ 0.01	$ 0.01
Issuance of preferred shares, net of costs		$ 0	$ 0	$ 144,991
Net cash proceeds of common stock		$ 143,027	$ 130,918	$ 118,583
At The Market Equity Program
Class of Stock [Line Items]
Shares issued		1,069,699	987,383
Aggregate offering price of common shares					$ 400,000
Net cash proceeds of common stock		$ 142,700	$ 126,100
Weighted average price per common share		$ 134.71	$ 129.19
Remaining capacity to issue		$ 128,300
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Commissions related to sales of common shares		1,200	$ 1,300
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, percentage	5.00%	5.00%	5.00%	5.00%
Preferred shares, par value	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Preferred shares, liquidation preference per share	$ 25,000	$ 25,000	$ 25,000	$ 25,000
Issuance of preferred shares, net of costs	$ 145,000
Preferred shares, shares outstanding		6,000	6,000
5.417% Series 1 Cumulative Convertible Preferred Shares
Class of Stock [Line Items]
Preferred shares, percentage		5.417%	5.417%	5.417%
Preferred shares, par value		$ 0.01	$ 0.01	$ 0.01
Preferred shares, liquidation preference per share		$ 25	$ 25	$ 25
Preferred shares, shares outstanding		399,896	399,896	399,896
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	$ 25	$ 25	$ 25
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
Preferred shares, shares outstanding		6,000,000	6,000,000	6,000,000