Common Shares (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	May 31, 2017	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 20, 2020
Treasury Stock, Shares, Acquired				3,222
Treasury Stock, Value, Acquired, Par Value Method				$ (253)
Number of common and preferred stock authorized to issue			185,000,000
Common shares, authorized			175,000,000	175,000,000
Preferred shares, authorized			10,000,000
Common Stock, Shares, Outstanding			97,200,000
Preferred Stock, Shares Outstanding			0
Common shares issued			3,599	0	4,778
Proceeds from issuance of common shares			$ 353,213	$ 0	$ 445,038
Cash distributions declared to equity holders per share			$ 3.20	$ 3.08	$ 3.00
2017 ATM program [Member]
Maximum aggregate offering price of common shares	$ 315,300
Common shares issued			224,300		28,100
2019 Equity Offering [Member]
Common shares issued			3,400,000
Proceeds from issuance of common shares			$ 328,400
Subsequent Event [Member]
Cash distributions declared to equity holders per share		$ 0.83
Subsequent Event [Member] | 2017 ATM program [Member]
Maximum aggregate offering price of remaining common shares available for sale						$ 287,700
April 2007 Repurchase Plan [Member]
Stock Repurchase Program, Authorized Amount			$ 500,000
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]
Stock Repurchase Program, Remaining Authorized Repurchase Amount						$ 269,500