Senior Unsecured Notes and Secured Debt (Details)		6 Months Ended
		Jun. 30, 2015 CAD	Jun. 30, 2015 GBP (£)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Principal payments due on debt obligations
2015	[1]			$ 284,750,000
2016				886,908,000
2017				850,621,000
2018				1,007,849,000
2019				1,673,543,000
Thereafter				6,439,662,000
Totals				11,143,333,000
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				8,060,493,000	$ 7,766,251,000
Secured debt				3,066,633,000	$ 2,977,713,000
Interest Rate margin		0.925%
Debt instrument maturity date		Oct. 31, 2018
Senior Unsecured Notes [Member]
Principal payments due on debt obligations
2015	[1],[2],[3]			56,975,000
2016	[2],[3]			400,000,000
2017	[2],[3]			450,000,000
2018	[2],[3]			450,000,000
2019	[2],[3],[4],[5]			1,300,417,000
Thereafter	[2],[3],[6],[7]			5,452,070,000
Totals	[2],[3]			8,109,462,000
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Notes, annual stated interest rates, Minimum		1.162%
Notes, annual stated interest rates, Maximum		6.50%
Secured Debt [Member]
Principal payments due on debt obligations
2015	[1],[2],[8]			227,775,000
2016	[2],[8]			486,908,000
2017	[2],[8]			400,621,000
2018	[2],[8]			557,849,000
2019	[2],[8]			373,126,000
Thereafter	[2],[8]			987,592,000
Totals	[2],[8]			3,033,871,000
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Notes, annual stated interest rates, Minimum		1.00%
Notes, annual stated interest rates, Maximum		7.98%
Carrying values of properties securing the debt				5,405,240,000
Canadian Denominated Unsecured Term Loan [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes		CAD 250,000,000		200,417,000
Interest Rate margin		2.00%
Debt Instrument, Issuance Date		Jul. 25, 2014
Debt instrument maturity date		Oct. 31, 2018
Canadian Denominated Unsecured Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest Rate margin		0.975%
Unsecured Line of Credit Term Loan [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				$ 350,000,000
UK Debt Due 2034 [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest rate		4.50%	4.50%	4.50%
Senior unsecured notes issued amount			£ 500,000,000	$ 786,700,000
Debt Instrument, Issuance Date		Nov. 25, 2014
Debt instrument maturity date		Nov. 24, 2034
UK Debt Due 2028 [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest rate		4.80%	4.80%	4.80%
Senior unsecured notes issued amount			£ 550,000,000	$ 865,370,000
Debt Instrument, Issuance Date		Nov. 20, 2013
Debt instrument maturity date		Nov. 20, 2028
Unsecured term credit facility [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				500,000,000
Interest Rate margin		1.15%
Debt Instrument, Issuance Date		Jul. 25, 2014
Debt instrument maturity date		Oct. 31, 2018
Unsecured term credit facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest Rate margin		0.975%
3.00% Senior Unsecured Convertible Notes due 2029 | Redemption 2015
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Debt Instrument, Redemption, Principle Amount Redeemed				$ 22,415,000
Debt Instrument, Redemption Period, End Date		Jul. 17, 2015
Debt Redemption Announcement Date		Aug. 17, 2015

[1] (4) Senior unsecured notes amount represents our 3.00% Convertible Senior Notes due 2029. On July 17, 2015, we announced that we will redeem all outstanding notes ($22,415,000, which amount does not include notes surrendered for conversion prior to July 17, 2015) on August 17, 2015.

[2] (1) Amounts represent principal amounts due and do not include unamortized premiums/discounts or other fair value adjustments as reflected on the balance sheet.

[3] (2) Annual interest rates range from 1.162% to 6.5%.

[4] (5) On July 25, 2014, we refinanced the funding on a $250,000,000 Canadian-denominated unsecured term credit facility (approximately $200,417,000 based on the Canadian/U.S. Dollar exchange rate on June 30, 2015). The loan matures on October 31, 2018 (with an option to extend for an additional year at our discretion) and bears interest at the Canadian Dealer Offered Rate plus 97.5 basis points (2.0% at June 30, 2015).

[5] (6) On July 25, 2014, we refinanced the funding on a $500,000,000 unsecured term credit facility. The loan matures on October 31, 2018 (with an option to extend for one additional year at our discretion) and bears interest at LIBOR plus 97.5 basis points (1.15% at June 30, 2015).

[6] (7) On November 20, 2013, we completed the sale of £550,000,000 (approximately $865,370,000 based on the Sterling/U.S. Dollar exchange rate in effect on June 30, 2015) of 4.8% senior unsecured notes due 2028.

[7] (8) On November 25, 2014, we completed the sale of £500,000,000 (approximately $786,700,000 based on the Sterling/U.S. Dollar exchange rate in effect on June 30, 2015) of 4.5% senior unsecured notes due 2034.

[8] (3) Annual interest rates range from 1.0% to 7.98%. Carrying value of the properties securing the debt totaled $5,405,240,000 at June 30, 2015.