CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (unaudited) - USD ($)	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 stock options 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	$ 1,764,360,000	$ 1,397,000	$ 40,000	$ 1,737,132,000	$ 25,791,000
Net income (loss) available to the Company	(13,878,000)	0	0	0	(13,878,000)
Compensation expense related to granting of restricted stock awards VALUE	3,913,000	(2,000)	0	3,915,000	0
Issuance of common stock related to the employee Stock Purchase Plan ("ESPP") VALUE	321,000	0	0	321,000	0
Exercise of stock options VALUE	13,000	0	0	13,000	0
Common stock repurchase VALUE	(19,379,000)	(18,000)	0	(19,361,000)	0
Purchase of vested employee restricted stock units	(3,463,000)	(3,000)	0	(3,460,000)	0
Payments of dividends on common stock VALUE	(13,036,000)	0	0	(13,036,000)	0
Dividend equivalents, net of forfeitures	342,000	0	0	342,000	0
Ending Balance VALUE at Mar. 31, 2018	1,719,193,000	$ 1,374,000	$ 40,000	1,705,866,000	11,913,000
Opening Balance SHARES at Dec. 31, 2017		139,675,781	4,045,199
Ending Balance SHARES at Sep. 30, 2018		138,482,653	4,045,199
Opening Balance VALUE at Dec. 31, 2017	1,764,360,000	$ 1,397,000	$ 40,000	1,737,132,000	25,791,000
Net income (loss) available to the Company	25,511,000
Net unrealized gain (loss) on derivatives VALUE	0
Ending Balance VALUE at Sep. 30, 2018	1,739,390,000	$ 1,386,000	$ 40,000	1,699,343,000	38,621,000
Opening Balance SHARES at Mar. 31, 2018		137,405,901	4,045,199
Compensation expense related to granting of stock options SHARES		1,198,734	0
Issuance of common stock related to an incentive plan SHARES		60,386	0
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	1,719,193,000	$ 1,374,000	$ 40,000	1,705,866,000	11,913,000
Net income (loss) available to the Company	2,441,000	0	0	0	2,441,000
Compensation expense related to granting of restricted stock awards VALUE	3,740,000	12,000	0	3,728	0
Issuance of common stock related to the employee Stock Purchase Plan ("ESPP") VALUE	388,000	1,000	0	387,000	0
Exercise of stock options VALUE	52,000	0	0	52,000	0
Purchase of vested employee restricted stock units	(1,709,000)	(2,000)	0	(1,707,000)	0
Payments of dividends on common stock VALUE	(12,746,000)	0	0	(12,746,000)	0
Dividend equivalents, net of forfeitures	127,000	0	0	127,000	0
Ending Balance VALUE at Jun. 30, 2018	1,711,486,000	$ 1,385,000	$ 40,000	1,695,707,000	14,354,000
Compensation expense related to granting of stock options SHARES		(96,321)	0
Issuance of common stock related to an incentive plan SHARES		50,785	0
Exercise of stock options SHARES		1,800	0
Purchase of vested employee restricted stock units SHARES		(857)	0
Ending Balance SHARES at Sep. 30, 2018		138,482,653	4,045,199
Net income (loss) available to the Company	36,948,000	$ 0	$ 0	0	36,948,000
Compensation expense related to granting of restricted stock awards VALUE	3,768,000	(1,000)	0	3,769,000	0
Issuance of common stock related to the employee Stock Purchase Plan ("ESPP") VALUE	342,000	1,000	0	341,000	0
Exercise of stock options VALUE	3,000	1,000	0	2,000	0
Purchase of vested employee restricted stock units	(7,000)	0	0	(7,000)	0
Payments of dividends on common stock VALUE	(12,491,000)	0	0	0	(12,491,000)
Dividend equivalents, net of forfeitures	(659,000)	0	0	(469,000)	(190,000)
Net unrealized gain (loss) on derivatives VALUE	0
Ending Balance VALUE at Sep. 30, 2018	1,739,390,000	$ 1,386,000	$ 40,000	1,699,343,000	38,621,000
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	1,334,260,000	$ 1,372,000	$ 40,000	1,693,512,000	(360,664,000)
Net income (loss) available to the Company	3,125,000	0	0	0	3,125,000
Compensation expense related to granting of restricted stock awards VALUE	3,573,000	14,000	0	3,559,000	0
Issuance of common stock related to the employee Stock Purchase Plan ("ESPP") VALUE	379,000	1,000	0	378,000	0
Exercise of stock options VALUE	244,000	2,000	0	242,000	0
Purchase of vested employee restricted stock units	(1,426,000)	(2,000)	0	(1,424,000)	0
Payments of dividends on common stock VALUE	(12,913,000)	0	0	(12,913,000)	0
Dividend equivalents, net of forfeitures	(463,000)	0	0	(463,000)	0
Application of amended leasing guidance	4,719,000	0	0	0	4,719,000
Ending Balance VALUE at Mar. 31, 2019	$ 1,331,498,000	$ 1,387,000	$ 40,000	1,682,891,000	(352,820,000)	$ 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 Sep. 30, 2019		133,583,672	4,045,199
Opening Balance VALUE at Dec. 31, 2018	$ 1,334,260,000	$ 1,372,000	$ 40,000	1,693,512,000	(360,664,000)
Net income (loss) available to the Company	67,323,000
Net unrealized gain (loss) on derivatives VALUE	(577,000)
Ending Balance VALUE at Sep. 30, 2019	1,367,870,000	$ 1,337,000	$ 40,000	1,655,690,000	(288,620,000)	(577,000)
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	1,331,498,000	$ 1,387,000	$ 40,000	1,682,891,000	(352,820,000)	0
Net income (loss) available to the Company	25,992,000	0	0	0	25,992,000	0
Compensation expense related to granting of restricted stock awards VALUE	3,393,000	0	0	3,393,000	0	0
Issuance of common stock related to the employee Stock Purchase Plan ("ESPP") VALUE	364,000	1,000	0	363,000	0	0
Purchase of vested employee restricted stock units	(1,300,000)	(2,000)	0	(1,298,000)	0	0
Payments of dividends on common stock VALUE	(13,140,000)	0	0	(13,140,000)	0	0
Dividend equivalents, net of forfeitures	1,059,000	0	0	1,059,000	0	0
Net unrealized gain (loss) on derivatives VALUE	(224,000)	0	0	0	0	(224,000)
Ending Balance VALUE at Jun. 30, 2019	1,347,642,000	$ 1,386,000	$ 40,000	1,673,268,000	(326,828,000)	(224,000)
Compensation expense related to granting of stock options SHARES		18,232	0
Issuance of common stock related to an incentive plan SHARES		100,965	0
Common stock repurchase SHARES		(5,000,000)	0
Purchase of vested employee restricted stock units SHARES		(1,408)	0
Ending Balance SHARES at Sep. 30, 2019		133,583,672	4,045,199
Net income (loss) available to the Company	38,208,000	$ 0	$ 0	0	38,208,000
Compensation expense related to granting of restricted stock awards VALUE	3,165,000	0	0	3,165,000	0	0
Issuance of common stock related to the employee Stock Purchase Plan ("ESPP") VALUE	288,000	1,000	0	287,000	0
Common stock repurchase VALUE	(18,340,000)	(50,000)	0	(18,290,000)	0
Purchase of vested employee restricted stock units	(4,000)	0	0	(4,000)	0	0
Payments of dividends on common stock VALUE	(2,684,000)	0	0	(2,684,000)	0
Dividend equivalents, net of forfeitures	(52,000)	0	0	(52,000)	0	0
Net unrealized gain (loss) on derivatives VALUE	(353,000)	0	0	0	0	(353,000)
Ending Balance VALUE at Sep. 30, 2019	$ 1,367,870,000	$ 1,337,000	$ 40,000	$ 1,655,690,000	$ (288,620,000)	$ (577,000)