Secured and unsecured senior debt (Details 5) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 rate	Sep. 30, 2011	Dec. 31, 2011 rate
Unsecured convertible notes
Principal amount	$ 3,033,123,000		$ 3,033,123,000
Unamortized discount	(979,000)		(979,000)
Components of interest cost recognized
Amortization of discount on liability component			401,000	3,254,000
8.00% Unsecured Senior Convertible Notes
Unsecured convertible notes
Principal amount	250,000		250,000		250,000
Unamortized discount	(10,000)		(10,000)		(15,000)
Net carrying amount of liability component	240,000		240,000		235,000
Carrying amount of equity component	27,000		27,000		27,000
Number of shares on which the aggregate consideration to be delivered on conversion is determined			6,087		6,087
Stated interest rate (as a percent)	8.00%		8.00%		8.00%
Effective interest rate (as a percent)			11.00%		11.00%
Conversion rate per $1,000 principal value of unsecured convertible notes			24.3480		24.3480
Components of interest cost recognized
Contractual interest	5,000	5,000	15,000	15,000
Amortization of discount on liability component	1,000	2,000	4,000	4,000
Total interest cost	6,000	7,000	19,000	19,000
3.70% Unsecured Senior Convertible Notes
Unsecured convertible notes
Principal amount					84,801,000
Unamortized discount					(77,000)
Net carrying amount of liability component					84,724,000
Carrying amount of equity component					8,080,000
Stated interest rate (as a percent)		3.70%		3.70%
Conversion price (in dollars per share)		$ 117.36		$ 117.36	$ 117.36
Denominator applied to principal	1,000		1,000
Closing stock price of common stock (in dollars per share)					$ 68.97
Components of interest cost recognized
Contractual interest		1,132,000	142,000	5,228,000
Amortization of discount on liability component		673,000	78,000	3,056,000
Total interest cost		$ 1,805,000	$ 220,000	$ 8,284,000