CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	(Distributions in Excess of Accumulated Earnings) Retained Earnings	Total Common Shareholders’ Equity	Noncontrolling Interests
Beginning Balance at Dec. 31, 2015	$ 1,521,354	$ 70	$ 1,092,239	$ (4,463)	$ 12,642	$ 1,100,488	$ 420,866
Balance (in Shares) at Dec. 31, 2015		70,258,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of OP Units to Common Shares by limited partners of the Operating Partnership	0	$ 1	7,874			7,875	(7,875)
Conversion of OP Units to Common Shares by limited partners of the Operating Partnership (in Shares)		350,000
Issuance of Common Shares, net of issuance costs	357,262	$ 10	357,252			357,262
Issuance of Common Shares, net of issuance costs (in Shares)		10,228,000
Issuance of OP Units to acquire real estate	29,336						29,336
Dividends declared ($0.78 and $0.75 per Common Share/OP Unit for Nine Months Ended September 30, 2017 and 2016)	(61,180)				(56,782)	(56,782)	(4,398)
Acquisition of noncontrolling interests	(18,379)		7,546			7,546	(25,925)
Employee and trustee stock compensation, net	11,682		699			699	10,983
Employee and trustee stock compensation, net (in Shares)		27,000
Change in control of previously unconsolidated investment	(75,713)						(75,713)
Noncontrolling interest distributions	(50,849)						(50,849)
Noncontrolling interest contributions	204,412						204,412
Reallocation of noncontrolling interests	0		35,254			35,254	(35,254)
Comprehensive income	91,128			(8,381)	52,955	44,574	46,554
Ending Balance at Sep. 30, 2016	2,009,053	$ 81	1,500,864	(12,844)	8,815	1,496,916	512,137
Balance (in Shares) at Sep. 30, 2016		80,863,000
Beginning Balance at Dec. 31, 2015	1,521,354	$ 70	1,092,239	(4,463)	12,642	1,100,488	420,866
Balance (in Shares) at Dec. 31, 2015		70,258,000
Ending Balance at Dec. 31, 2016	$ 2,178,125	$ 84	1,594,926	(798)	(5,635)	1,588,577	589,548
Balance (in Shares) at Dec. 31, 2016	83,597,741	83,598,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of OP Units to Common Shares by limited partners of the Operating Partnership	$ 0		1,086			1,086	(1,086)
Conversion of OP Units to Common Shares by limited partners of the Operating Partnership (in Shares)		61,000
Dividends declared ($0.78 and $0.75 per Common Share/OP Unit for Nine Months Ended September 30, 2017 and 2016)	(70,053)				(65,248)	(65,248)	(4,805)
Employee and trustee stock compensation, net	9,129		425			425	8,704
Employee and trustee stock compensation, net (in Shares)		21,000
Noncontrolling interest distributions	(7,278)						(7,278)
Noncontrolling interest contributions	20,522						20,522
Reallocation of noncontrolling interests	0		(2,105)			(2,105)	2,105
Comprehensive income	39,349			245	40,558	40,803	(1,454)
Ending Balance at Sep. 30, 2017	$ 2,169,794	$ 84	$ 1,594,332	$ (553)	$ (30,325)	$ 1,563,538	$ 606,256
Balance (in Shares) at Sep. 30, 2017	83,680,337	83,680,000