Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Class A Common Stock	Class B Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Additional Paid- in-Capital	Treasury Stock	Retained Earnings/(Dividends in Excess of Earnings)	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests Operating Partnership	Noncontrolling Interests in Consolidated Joint Ventures
Beginning Balance (in shares) at Dec. 31, 2016				71,586,000	38,003,000
Beginning Balance at Dec. 31, 2016	$ 1,509,554			$ 72	$ 38	$ 992,307	$ (11,244)	$ (11,148)	$ 1,365	$ 533,246	$ 4,918
Increase Decrease in Stockholders' Equity
Exchange of noncontrolling interest for common stock (in shares)		13,737,365	13,737,365
Net income (loss)	50,688
Other comprehensive income (loss)	5,181								3,018
Ending Balance at Jun. 30, 2017									5,703
Beginning Balance (in shares) at Dec. 31, 2016				71,586,000	38,003,000
Beginning Balance at Dec. 31, 2016	1,509,554			$ 72	$ 38	992,307	(11,244)	(11,148)	1,365	533,246	4,918
Increase Decrease in Stockholders' Equity
Contributions	7,479									0	7,479
Distributions	(42,524)									(42,218)	(306)
Equity based compensation	18,965					18,965
Grants of restricted stock (in shares)				1,997,000
Grants of restricted stock	0			$ 1		(1)
Purchase of treasury stock (in shares)				(190,000)
Purchase of treasury stock	(2,588)						(2,588)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock and units (in shares)				(1,323,000)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock and units	(18,125)			$ (1)			(18,124)
Forfeitures (in shares)				(10,000)
Forfeitures	0
Dividends declared	(105,921)							(105,921)
Stock dividends (in shares)				814,000	432,000
Stock dividends	0			$ 1	$ 1	17,317		(17,319)
Exchange of noncontrolling interest for common stock (in shares)				20,767,000	20,767,000
Exchange of noncontrolling interest for common stock	(2,353)			$ 21	$ (21)	280,714			1,696	(284,763)
Net income (loss)	125,879							95,276		30,377	226
Other comprehensive income (loss)	(2,220)								(2,915)	695
Rebalancing of ownership percentage between Company and Operating Partnership	0					(3,166)			(358)	3,524
Ending Balance (in shares) at Dec. 31, 2017				93,641,000	17,668,000
Ending Balance at Dec. 31, 2017	1,488,146			$ 94	$ 18	1,306,136	(31,956)	(39,112)	(212)	240,861	12,317
Increase Decrease in Stockholders' Equity
Contributions	5,040									0	5,040
Distributions	(22,691)									(8,899)	(13,792)
Equity based compensation	4,505					4,505
Grants of restricted stock (in shares)				29,000
Grants of restricted stock	0			$ 0		0
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock and units (in shares)				(56,000)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock and units	(837)			$ 0			(837)
Forfeitures (in shares)				(26,000)
Forfeitures	0
Dividends declared	(62,275)							(62,275)
Exchange of noncontrolling interest for common stock (in shares)		4,349,832	4,349,832	4,350,000	4,350,000
Exchange of noncontrolling interest for common stock	313			$ 5	$ (5)	60,110			(143)	(59,654)
Net income (loss)	111,365							89,281		13,795	8,289
Other comprehensive income (loss)	(11,104)								(9,506)	(1,598)
Rebalancing of ownership percentage between Company and Operating Partnership	0					(659)			6	653
Ending Balance (in shares) at Jun. 30, 2018				97,938,000	13,318,000
Ending Balance at Jun. 30, 2018	$ 1,512,462			$ 99	$ 13	$ 1,370,092	$ (32,793)	$ (12,106)	$ (9,855)	$ 185,158	$ 11,854