Indebtedness (Details) (USD $) In Thousands, unless otherwise specified			3 Months Ended								1 Months Ended
	Mar. 31, 2014 item	Dec. 31, 2013	Mar. 31, 2014 Unsecured revolving credit facility	May 02, 2014 Unsecured revolving credit facility Subsequent event	Mar. 31, 2014 Senior unsecured notes 4.30%	Mar. 31, 2014 Senior unsecured notes 6.75%	Mar. 31, 2014 Senior unsecured notes 6.75% maturing in 2021	Mar. 31, 2014 Senior unsecured notes 5.625% due in 2042	Mar. 31, 2014 Mortgages building property	Mar. 31, 2014 Capital leases property	Apr. 30, 2014 Senior unsecured notes due 2019 Subsequent event	Apr. 30, 2014 Senior unsecured notes due 2024 Subsequent event
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity	$ 750		$ 750,000
Number of public issues of unsecured senior notes	4
Principal amount of unsecured senior notes	1,093,658	1,093,337			250,000	200,000	300,000	350,000
Aggregate principal amount of mortgage debt									678,514
Interest rate (as a percent)					4.30%	6.75%	6.75%	5.625%			3.25%	4.75%
Number of buildings mortgaged									51
Number of properties mortgaged									48
Number of properties recorded under capital lease										2
Carrying value of mortgaged properties	4,410,178	4,422,865							941,064	18,534
Capital leases										13,181
Extension period of revolving credit facility			1 year
Principal repayment due until maturity			0
Option to increase the borrowing capacity under revolving credit facility			1,500,000
Interest rate, description			LIBOR
Interest rate added to the base rate (as a percent)			1.30%
Debt instrument, facility fee (as a percent)			0.30%
Weighted average interest rate on debt (as a percent)			1.42%
Interest expense and other associated costs incurred			425
Unsecured revolving credit facility, amount outstanding			145,000	0
Revolving credit facility, available amount			605,000	750,000
Sale of senior unsecured notes											400,000	250,000
Net proceeds after underwriting discounts from sale of senior unsecured notes												$ 644,889