Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended		12 Months Ended			0 Months Ended	1 Months Ended					0 Months Ended	1 Months Ended
	Oct. 31, 2014	Apr. 30, 2014	Dec. 31, 2014 item	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2013	Jun. 30, 2014 item	Sep. 30, 2013 item	Jun. 30, 2013 item	Feb. 28, 2015 property	Dec. 31, 2014 property	May 30, 2014	May 31, 2014	Feb. 26, 2015	Jul. 31, 2012	Dec. 31, 2011	Aug. 31, 2012 item
Indebtedness
Unsecured revolving credit facility, maximum borrowing capacity			$ 750,000								$ 750,000
Number of public issues of unsecured senior notes			6								6
Principal amount of unsecured senior notes			1,743,628	1,093,337							1,743,628
Unsecured term loan facility			1,750,000	1,100,000							1,750,000
Aggregate principal amount of mortgage debt			611,369								611,369
Interest rate (as a percent)	5.875%
Carrying value of mortgaged properties			5,254,761	4,422,865							5,254,761
Assumption of mortgage notes payable			15,630	12,266	121,793
Repayment of debt	14,700
Loss on early extinguishment of debt			(12)	(797)	(6,349)
Sale of senior unsecured notes			350,000
Net proceeds after underwriting discounts from sale of senior unsecured notes		644,889
Unsecured revolving credit facility			80,000	100,000							80,000
Net book value of the assets encumbered by capital leases			18,237	18,627							18,237
Weighted average interest rate on debt (as a percent)			6.28%	6.25%							6.28%
Unsecured revolving credit facility
Indebtedness
Extension period of revolving credit facility			1 year
Option to increase the borrowing capacity under revolving credit facility			1,500,000								1,500,000
Interest rate, description				LIBOR		LIBOR
Interest rate added to the base rate (as a percent)				1.30%		1.60%
Debt instrument, facility fee (as a percent)				0.30%		0.35%
Revolving credit facility, interest rate payable (as a percent)			1.40%								1.40%
Weighted average interest rate on debt (as a percent)			1.40%	1.60%	1.80%
Loss on early extinguishment of debt				538
Principal repayment due until maturity			0
Interest expense and other associated costs incurred			3,094	3,781	5,733
Unsecured revolving credit facility			80,000								80,000			0
Revolving credit facility, available amount			750,000								750,000
Unsecured revolving credit facility | Subsequent event
Indebtedness
Revolving credit facility, available amount			750,000								750,000
Senior unsecured notes 4.30%
Indebtedness
Principal amount of unsecured senior notes			250,000								250,000
Unsecured term loan facility			250,000	250,000							250,000
Interest rate (as a percent)			4.30%								4.30%
Senior unsecured notes 6.75%
Indebtedness
Principal amount of unsecured senior notes			200,000								200,000
Unsecured term loan facility			200,000	200,000							200,000				350,000	300,000
Interest rate (as a percent)			6.75%								6.75%
Unsecured senior notes due 2021
Indebtedness
Principal amount of unsecured senior notes			300,000								300,000
Unsecured term loan facility			300,000	300,000							300,000
Interest rate (as a percent)			6.75%								6.75%
Senior unsecured notes 5.625% due in 2042
Indebtedness
Principal amount of unsecured senior notes			350,000								350,000
Unsecured term loan facility			350,000	350,000							350,000
Interest rate (as a percent)			5.625%								5.625%
Mortgages
Indebtedness
Aggregate principal amount of mortgage debt			611,369								611,369
Number of properties mortgaged			45								45						28
Carrying value of mortgaged properties			816,065								816,065
Number of mortgage notes							2	2	4
Repayment of debt							35,807	13,579	10,377
Loss on early extinguishment of debt			12	259
Number of Properties released from mortgage as collateral																	11
Number of buildings mortgaged			47								47
Weighted average interest rate on debt (as a percent)							5.80%	6.90%	6.10%
Mortgages | Significant Unobservable Inputs (Level 3)
Indebtedness
Interest rate (as a percent)			6.28%								6.28%
Assumption of mortgage notes payable			15,630
Fair value of mortgage notes payable			16,643								16,643
Increase (decrease) in outstanding principal balances			1,013
Mortgages | Subsequent event
Indebtedness
Interest rate (as a percent)	6.25%		6.37%							6.02%	6.37%
Repayment of debt	11,900									29,227	11,308
Number of properties encumbered under mortgage notes	1		1							1	1
Capital leases
Indebtedness
Interest rate (as a percent)			7.70%								7.70%
Number of properties mortgaged			2								2
Carrying value of mortgaged properties			18,237	18,627							18,237
Number of properties recorded under capital lease			2								2
Capital leases			12,770								12,770
Senior unsecured notes due 2019
Indebtedness
Principal amount of unsecured senior notes		400,000	400,000								400,000
Interest rate (as a percent)		3.25%	3.25%								3.25%
Senior unsecured notes due 2024
Indebtedness
Principal amount of unsecured senior notes		250,000	250,000								250,000
Interest rate (as a percent)		4.75%	4.75%								4.75%
Unsecured Debt
Indebtedness
Unsecured term loan facility			350,000								350,000		350,000
Interest rate (as a percent)			1.60%								1.60%
Interest rate, description												LIBOR
Interest rate added to the base rate (as a percent)												1.40%	1.40%
Interest expense and other associated costs incurred			3,263
Maximum borrowing capacity that may be increased												700,000
Weighted average interest rate on debt (as a percent)			1.60%								1.60%
Senior notes issued in April 2014
Indebtedness
Net proceeds after underwriting discounts from sale of senior unsecured notes		$ 644,889