Earnings Per Common Share (Schedule Of Net Income (Loss) Per Common Share) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Common Share [Abstract]
Net income (loss)	$ 243,057,000	$ 104,398,000	$ 450,549,000	$ 791,817,000
Less: dividends declared-common shares and unnvested common shares	(61,910,000)	(56,995,000)	(187,115,000)	(171,131,000)
Undistributed earnings	$ 181,148,000	$ 47,403,000	$ 263,435,000	$ 620,687,000
Percentage allocated to common shareholders	98.80%	98.90%	98.70%	98.90%
Net income (loss) available to common stockholders, total	$ 178,938,000	$ 46,869,000	$ 260,096,000	$ 613,847,000
Add: dividends declared-common shareholders	61,199,000	56,403,000	184,836,000	169,329,000
Numerator for basic and diluted earnings per common share	$ 240,138,000	$ 103,273,000	$ 444,931,000	$ 783,176,000
Denominator for basic earnings per weighted-average common shares	39,483,000	40,287,000	39,711,000	40,289,000
Effect of dilutive securities: Options	74,000	125,000	79,000	131,000
Denominator for diluted earnings per adjusted weighted-average common shares	39,557,000	40,411,000	39,790,000	40,421,000
Per common share net income (loss)
Basic	$ 6.08	$ 2.56	$ 11.20	$ 19.44
Diluted	$ 6.07	$ 2.56	$ 11.18	$ 19.38
Basic weighted-average common shares outstanding	39,483,000	40,287,000	39,711,000	40,289,000
Basic weighted-average common shares outstanding and unnvested common shares expected to vest	39,971,000	40,746,000	40,221,000	40,738,000