Secured and unsecured senior debt (Details 6) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended		9 Months Ended
	Apr. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Apr. 30, 2012 3.70% Unsecured Senior Convertible Notes	Jan. 31, 2012 3.70% Unsecured Senior Convertible Notes	Sep. 30, 2011 3.70% Unsecured Senior Convertible Notes
Unsecured convertible notes
Stated interest rate (as a percent)								3.70%
Principal amount of unsecured convertible notes repurchased	$ 1,000,000					$ 1,000,000	$ 83,800,000	$ 217,100,000
Loss on early extinguishment of debt			2,742,000	2,225,000	6,485,000			5,200,000
Interest expense incurred
Gross interest		33,857,000	30,939,000	99,097,000	93,591,000
Capitalized interest		(16,763,000)	(16,666,000)	(47,854,000)	(44,905,000)
Interest expense		$ 17,094,000	$ 14,273,000	$ 51,243,000	$ 48,686,000