Commitments and Contingencies (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Leases with purchase options, annualized base rent receivables in next five years and thereafter
Leases with purchase option, annualized base rent receivables in 2013	$ 42,700,000
Leases with purchase option, annualized base rent receivables in 2014	36,666,000
Leases with purchase option, annualized base rent receivables in 2015	16,702,000
Leases with purchase option, annualized base rent receivables in 2016	38,933,000
Leases with purchase option, annualized base rent receivables in 2017	1,685,000
Leases with purchase option, annualized base rent receivables thereafter	96,859,000
Total leases with purchase option, annualized base rent receivables	233,545,000
Number of properties with purchase options, annualized base rent receivables
Number of properties with a purchase option, annualized base rent receivables in 2013	23
Number of properties with a purchase option, annualized base rent receivables in 2014	15
Number of properties with a purchase option, annualized base rent receivables in 2015	15
Number of properties with a purchase option, annualized base rent receivables in 2016	18
Number of properties with a purchase option, annualized base rent receivables in 2017	2
Number of properties with a purchase option, annualized base rent receivables thereafter	58
Total number of properties with a purchase option, annualized base rent receivables	131
Rental Expense
Rental expense attributable to continuing operations	7,000,000	6,000,000	6,000,000
Term of ground leases	99 years
Future minimum lease obligations under non-cancelable ground and other operating leases
Future minimum lease obligations under non-cancelable ground and other operating leases, 2013	7,734,000
Future minimum lease obligations under non-cancelable ground and other operating leases, 2014	7,119,000
Future minimum lease obligations under non-cancelable ground and other operating leases, 2015	6,372,000
Future minimum lease obligations under non-cancelable ground and other operating leases, 2016	5,228,000
Future minimum lease obligations under non-cancelable ground and other operating leases, 2017	4,797,000
Future minimum lease obligations under non-cancelable ground and other operating leases, Thereafter	193,324,000
Future minimum lease obligations under non-cancelable ground and other operating leases, Total	$ 224,574,000