Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Mar. 07, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 04, 2016
Class of Stock [Line Items]
Shares issued under dividend reinvestment plan (in shares)		15,619	16,524	18,705
Preferred shares, shares outstanding		399,896	399,896	399,896
Series 1 Cumulative Convertible Preferred interest rate		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
Net cash proceeds of common stock		$ 329,103	$ 110,855	$ 216,155
Public Equity Offering
Class of Stock [Line Items]
Common shares issued	1,000,000
Price per share	$ 149.43
Net cash proceeds of common stock	$ 149,300
At The Market Equity Program
Class of Stock [Line Items]
Common shares issued		1,156,571	813,414
Net cash proceeds of common stock		$ 174,800	$ 108,500
Aggregate offering price of common shares					$ 400,000
Weighted average price per common share		$ 152.92	$ 135.01
Remaining capacity to issue		$ 370,900
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Commissions related to sales of common shares		1,800	$ 1,100
At The Market Equity Program | Other Offering Costs
Class of Stock [Line Items]
Commissions related to sales of common shares		$ 200	$ 200