Net Income (Loss) Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Numerator [Abstract]
Net income (loss)	$ 111,500	$ 486,700	$ 176,100	$ 360,100	$ (17,600)	$ 180,100	$ 124,200	$ (807,000)	$ 1,134,514		$ (520,291)		$ 852,552
Dividends on preferred shares	(15,400)	(15,400)	(15,400)	(15,400)	(15,400)	(14,400)	(8,600)	(8,600)	(61,622)		(47,020)		(34,525)
Net income (loss) available to common shareholders	$ 96,100	$ 471,300	$ 160,700	$ 344,700	$ (33,000)	$ 165,700	$ 115,600	$ (815,600)	$ 1,072,892		$ (567,311)		$ 818,027
Denominator [Abstract]
Weighted average number of common shares outstanding									62,915,992		67,558,732		76,839,519
Share options and other									699,700	[1]	0	[1]	1,394,800	[1]
Weighted average number of common shares and common share equivalents outstanding									63,615,748		67,558,732		78,234,312
Basic net income (loss)	$ 1.58	$ 7.62	$ 2.52	$ 5.27	$ (0.49)	$ 2.45	$ 1.71	$ (11.99)	$ 17.05		$ (8.40)		$ 10.65
Diluted net income (loss)	$ 1.56	$ 7.53	$ 2.50	$ 5.24	$ (0.49)	$ 2.43	$ 1.69	$ (11.99)	$ 16.87	[1]	$ (8.40)	[1]	$ 10.46	[1]
Share options excluded from computation of weighted average number of common share and common share equivalents outstanding									554,700		2,854,400		489,700
Weighted average number of potentially dilutive common shares and common share equivalents outstanding											687,300

[1] At December 31, 2012, 2011 and 2010, share based awards to purchase 554.7 thousand, 2,854.4 thousand and 489.7 thousand common shares, respectively, were excluded from the calculation of diluted weighted average number of common shares and common share equivalents outstanding because their exercise prices were greater than the average market price of the common shares. In addition, dilutive securities, in the form of share options and other, of 687.3 thousand shares were not included in the weighted average number of common shares and common share equivalents outstanding for the purpose of computing the diluted net loss per share because to do so would have been anti-dilutive for year ended December 31, 2011.