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)			6,790,121	(6,790,121)
Exchange of noncontrolling interest for common stock										403
Net income (loss)	19,630
Other comprehensive income (loss)	4,751									3,117
Ending Balance at Mar. 31, 2017										4,834
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	2,635										0	2,635
Distributions	(17,993)										(4,273)	(13,720)
Amendment of the par value of the Class B shares from no par value per share to $0.001 per share	0							0
Equity based compensation	2,400						2,400
Grants of restricted stock (in shares)					25,000
Grants of restricted stock	0				$ 0		0
Purchase of treasury stock (in shares)					0
Purchase of treasury stock	0							0
Re-issuance of treasury stock (in shares)					0
Re-issuance of treasury stock	0							0
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock and units (in shares)					(49,000)
Shares acquired to satisfy minimum required federal and state tax withholding on vesting restricted stock and units	(728)				$ 0			(728)
Forfeitures (in shares)					(11,000)
Forfeitures	0
Dividends declared	(30,422)								(30,422)
Stock dividends (in shares)					0	0
Stock dividends	0				$ 0	$ 0	0			0	0
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)
Adjustment for deferred taxes/tax receivable agreement as a result of the exchange of Class B shares	0
Net income (loss)	67,798								50,875		8,501	8,422
Other comprehensive income (loss)	(8,857)									(7,528)	(1,329)
Rebalancing of ownership percentage between Company and Operating Partnership	0						(98)			3	95
Ending Balance (in shares) at Mar. 31, 2018					97,956,000	13,318,000
Ending Balance at Mar. 31, 2018	$ 1,503,292	[1]			$ 99	$ 13	$ 1,368,548	$ (32,684)	$ (18,659)	$ (7,880)	$ 184,201	$ 9,654

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.