Earnings Per Common Share (Schedule Of Net Income (Loss) Per Common Share) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Common Share [Abstract]
Net income (loss)	$ 190,880,000	$ 332,868,000	$ 207,492,000	$ 687,419,000
Less: dividends declared-common shares and nonvested common shares	(61,927,000)	(56,999,000)	(125,205,000)	(114,136,000)
Undistributed earnings	$ 128,953,000	$ 275,869,000	$ 82,287,000	$ 573,283,000
Percentage allocated to common shareholders	98.70%	98.90%	98.70%	98.90%
Net income (loss) available to common stockholders, total	$ 127,233,000	$ 272,750,000	$ 81,224,000	$ 567,037,000
Add: dividends declared-common shareholders	61,148,000	56,394,000	123,637,000	112,925,000
Numerator for basic and diluted earnings per common share	$ 188,381,000	$ 329,144,000	$ 204,861,000	$ 679,962,000
Denominator for basic earnings per weighted-average common shares	39,449,000	40,277,000	39,827,000	40,291,000
Effect of dilutive securities: Options	69,000	126,000	81,000	134,000
Denominator for diluted earnings per adjusted weighted-average common shares	39,519,000	40,404,000	39,908,000	40,425,000
Basic	$ 4.78	$ 8.17	$ 5.14	$ 16.88
Diluted	$ 4.77	$ 8.15	$ 5.13	$ 16.82
Basic weighted-average common shares outstanding	39,449,000	40,277,000	39,827,000	40,291,000
Basic weighted-average common shares outstanding and nonvested common shares expected to vest	39,983,000	40,738,000	40,348,000	40,735,000