Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 29, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Net cash proceeds of common stock				$ 51,189	$ 300,040
Preferred shares, percentage		5.417%	5.417%	5.417%	5.417%
Preferred shares, par value		$ 0.01		$ 0.01		$ 0.01
Net cash proceeds of preferred shares				$ 145,456	$ 0
At The Market Equity Program
Class of Stock [Line Items]
Aggregate offering price of common shares		$ 400,000		$ 400,000
Weighted average price per common share		$ 131.00		$ 133.09
Net cash proceeds of common stock		$ 29,300		$ 42,700
Remaining capacity to issue		327,600		327,600
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Payments of stock issuance costs		300		400
At The Market Equity Program | Other offering costs
Class of Stock [Line Items]
Payments of stock issuance costs		$ 100		$ 100
Common Shares
Class of Stock [Line Items]
Shares issued (in shares)				325,451
Common Shares | At The Market Equity Program
Class of Stock [Line Items]
Shares issued (in shares)		226,739		325,397
Preferred Shares
Class of Stock [Line Items]
Shares issued (in shares)				6,000
5.0% Series C Cumulative Redeemable Preferred
Class of Stock [Line Items]
Preferred shares, percentage	5.00%			5.00%
Preferred shares, par value	$ 0.01
Preferred shares, liquidation preference	$ 25,000	$ 25,000		$ 25,000
Net cash proceeds of preferred shares	$ 145,000
Depository Shares | 5.0% Series C Cumulative Redeemable Preferred
Class of Stock [Line Items]
Preferred shares, liquidation preference	$ 25
Depository Shares | 5.0% Series C Cumulative Redeemable Preferred | Preferred Shares
Class of Stock [Line Items]
Shares issued (in shares)	6,000,000