Earnings per Share (Details) (USD $) In Thousands, except Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Reconciliation of basic and diluted net income and distributions per common share
Net income	$ 39,820	$ 82,342	$ 241,091	$ 292,865
Less: Dividends and undistributed earnings allocated to participating securities	99	687	921	2,482
Net income applicable to common shareholders	$ 39,721	$ 81,655	$ 240,170	$ 290,383
Basic:
Basic weighted average shares outstanding (in shares)	177,723,000	157,057,000	177,493,000	157,018,000
Net income per share (in dollars per share)	$ 0.22	$ 0.52	$ 1.35	$ 1.85
Diluted:
Basic weighted average shares outstanding (in shares)	177,723,000	157,057,000	177,493,000	157,018,000
Dilutive effect of convertible senior notes	2,497,000	404,000	3,811,000	276,000
Diluted weighted average shares outstanding (in shares)	180,220,000	157,461,000	181,304,000	157,294,000
Net income per share (in dollars per share)	$ 0.22	$ 0.52	$ 1.32	$ 1.85
Dividends
Distributions declared per common share (in dollars per share)	$ 0.18	$ 0.12	$ 0.49	$ 0.29
Anti-dilutive securities
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	1,932,279	1,932,279	1,932,279	1,932,279