CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (unaudited) - USD ($) $ in Thousands	Total	Common Class A [Member]	Common Class B [Member	Additional Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Opening Balance SHARES at Dec. 31, 2017		139,675,781	4,045,199
Compensation expense related to granting of restricted stock awards SHARES		(157,680)	0
Issuance of common stock related to an incentive plan SHARES		39,196	0
Common stock repurchase SHARES		(1,833,200)	0
Exercise of stock options SHARES		10,000	0
Purchase of vested employee restricted stock units SHARES		(328,196)	0
Ending Balance SHARES at Mar. 31, 2018		137,405,901	4,045,199
Opening Balance VALUE at Dec. 31, 2017	$ 1,764,360	$ 1,397	$ 40	$ 1,737,132	$ 25,791
NET INCOME (LOSS)	(13,878)	0	0	0	(13,878)
Compensation expense related to granting of restricted stock awards VALUE	3,913	(2)	0	3,915	0
Issuance of common stock related to an incentive plan VALUE	321	0	0	321	0
Exercise of stock options VALUE	13	0	0	13	0
Common stock repurchase VALUE	(19,379)	(18)	0	(19,361)	0
Purchase of vested employee restricted stock units VALUE	3,463	3	0	3,460	0
Payments of dividends on common stock VALUE	(13,036)	0	0	(13,036)	0
Dividend equivalents, net of forfeitures	342	0	0	342	0
Ending Balance VALUE at Mar. 31, 2018	1,719,193	$ 1,374	$ 40	1,705,866	11,913
Opening Balance SHARES at Dec. 31, 2017		139,675,781	4,045,199
Ending Balance SHARES at Jun. 30, 2018		138,527,246	4,045,199
Opening Balance VALUE at Dec. 31, 2017	1,764,360	$ 1,397	$ 40	1,737,132	25,791
NET INCOME (LOSS)	(11,437)
Purchase of vested employee restricted stock units VALUE	5,172
Net unrealized gain (loss) on derivatives VALUE	0
Ending Balance VALUE at Jun. 30, 2018	1,711,486	$ 1,385	$ 40	1,695,707	14,354
Opening Balance SHARES at Mar. 31, 2018		137,405,901	4,045,199
Compensation expense related to granting of restricted stock awards SHARES		1,198,734	0
Issuance of common stock related to an incentive plan SHARES		60,386
Exercise of stock options SHARES		38,500	0
Purchase of vested employee restricted stock units SHARES		(176,275)	0
Ending Balance SHARES at Jun. 30, 2018		138,527,246	4,045,199
Opening Balance VALUE at Mar. 31, 2018	1,719,193	$ 1,374	$ 40	1,705,866	11,913
NET INCOME (LOSS)	2,441	0	0	0	2,441
Compensation expense related to granting of restricted stock awards VALUE	3,740	12	0	3,728	0
Issuance of common stock related to an incentive plan VALUE	388	1	0	387	0
Exercise of stock options VALUE	52	0	0	52	0
Purchase of vested employee restricted stock units VALUE	1,709	2	0	1,707	0
Payments of dividends on common stock VALUE	(12,746)	0	0	(12,746)	0
Dividend equivalents, net of forfeitures	127	0	0	127	0
Net unrealized gain (loss) on derivatives VALUE	0
Ending Balance VALUE at Jun. 30, 2018	1,711,486	$ 1,385	$ 40	1,695,707	14,354
Opening Balance SHARES at Dec. 31, 2018		137,180,213	4,045,199
Compensation expense related to granting of stock options SHARES		1,406,722	0
Issuance of common stock related to an incentive plan SHARES		84,958	0
Exercise of stock options SHARES		180,300	0
Purchase of vested employee restricted stock units SHARES		(204,499)	0
Ending Balance SHARES at Mar. 31, 2019		138,647,694	4,045,199
Opening Balance VALUE at Dec. 31, 2018	1,334,260	$ 1,372	$ 40	1,693,512	(360,664)
NET INCOME (LOSS)	3,125	0	0	0	3,125
Compensation expense related to granting of stock options VALUE	3,573	14	0	3,559	0
Issuance of common stock related to an incentive plan VALUE	379	1	0	378	0
Exercise of stock options VALUE	244	2	0	242	0
Purchase of vested employee restricted stock units VALUE	1,426	2	0	1,424	0
Payments of dividends on common stock VALUE	(12,913)	0	0	(12,913)	0
Forfeitures of dividend equivalents VALUE	(463)	0	0	(463)	0
Application of amended leasing guidance	4,719	0	0	0	4,719
Ending Balance VALUE at Mar. 31, 2019	$ 1,331,498	$ 1,387	$ 40	1,682,891	(352,820)	$ 0
Opening Balance SHARES at Dec. 31, 2018		137,180,213	4,045,199
Compensation expense related to granting of stock options SHARES	0
Exercise of stock options SHARES	180,300
Ending Balance SHARES at Jun. 30, 2019		138,465,883	4,045,199
Opening Balance VALUE at Dec. 31, 2018	$ 1,334,260	$ 1,372	$ 40	1,693,512	(360,664)
NET INCOME (LOSS)	29,116
Purchase of vested employee restricted stock units VALUE	2,726
Net unrealized gain (loss) on derivatives VALUE	(224)
Ending Balance VALUE at Jun. 30, 2019	1,347,642	$ 1,386	$ 40	1,673,268	(326,828)	(224)
Opening Balance SHARES at Mar. 31, 2019		138,647,694	4,045,199
Compensation expense related to granting of stock options SHARES		(38,774)	0
Issuance of common stock related to an incentive plan SHARES		73,791	0
Purchase of vested employee restricted stock units SHARES		(216,828)	0
Ending Balance SHARES at Jun. 30, 2019		138,465,883	4,045,199
Opening Balance VALUE at Mar. 31, 2019	1,331,498	$ 1,387	$ 40	1,682,891	(352,820)	0
NET INCOME (LOSS)	25,992	0	0	0	25,992	0
Compensation expense related to granting of stock options VALUE	3,393	0	0	3,393	0	0
Issuance of common stock related to an incentive plan VALUE	364	1	0	363	0	0
Purchase of vested employee restricted stock units VALUE	1,300	2	0	1,298	0	0
Payments of dividends on common stock VALUE	(13,140)	0	0	(13,140)	0	0
Dividend equivalents, net of forfeitures	1,059	0	0	1,059	0	0
Net unrealized gain (loss) on derivatives VALUE	(224)	0	0	0	0	(224)
Ending Balance VALUE at Jun. 30, 2019	$ 1,347,642	$ 1,386	$ 40	$ 1,673,268	$ (326,828)	$ (224)