Secured and unsecured senior debt (Details 6) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended		1 Months Ended		6 Months Ended
	Apr. 30, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Apr. 30, 2012 3.70% Unsecured Senior Convertible Notes	Jan. 31, 2012 3.70% Unsecured Senior Convertible Notes	Jun. 30, 2011 3.70% Unsecured Senior Convertible Notes	Dec. 31, 2011 3.70% Unsecured Senior Convertible Notes
Unsecured convertible notes
Principal amount		$ 2,999,850,000		$ 2,999,850,000					$ 84,801,000
Stated interest rate (as a percent)								3.70%
Principal amount of unsecured convertible notes repurchased	1,000,000					1,000,000	83,800,000	96,100,000
Loss on early extinguishment of debt		1,602,000	1,248,000	2,225,000	3,743,000			2,500,000
Repurchase of unsecured convertible notes, aggregate cash price				84,801,000	98,590,000
Interest expense incurred
Gross interest		33,747,000	31,617,000	65,240,000	62,652,000
Capitalized interest		(15,825,000)	(15,046,000)	(31,091,000)	(28,239,000)
Interest expense		$ 17,922,000	$ 16,567,000	$ 34,149,000	$ 34,377,000