Senior Unsecured Notes and Secured Debt (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Unsecured note issuances
Senior unsecured debt issued	$ 497,862	$ 0
Senior unsecured debt redeemed	0	22
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	6,394,403	4,464,905	5,894,403	4,464,927
Senior unsecured debt issued	500,000	0
Senior unsecured debt extinguished	0	0
Senior unsecured debt redeemed	$ 0	$ (22)
Senior unsecured debt balance average rate	0.04431	0.05133	0.04675	0.05133
Senior unsecured debt issued average rate	0.01552	0.000
Senior unsecured debt extinguished average rate	0.000	0.000
Senior unsecured debt redeemed average rate	0.000	0.04750