Senior Unsecured Notes and Secured Debt (Details) $ / shares in Units, £ in Thousands, CAD in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 CAD	Dec. 31, 2015 GBP (£)	Dec. 31, 2015 USD ($) $ / shares
Principal payments due on debt obligations
2016							$ 947,325,000
2017							926,661,000
2018							1,100,763,000
2019	[1],[2]						1,661,237,000
2020	[3]						840,612,000
Thereafter	[4],[5],[6]						6,647,367,000
Totals							12,123,965,000
Senior Unsecured Convertible Notes Issued [Abstract]
Equity component of convertible debt, reacquire		$ 6,761,000		$ (555,000)
Gains (Losses) on Extinguishment of Debt		$ (34,677,000)	$ (9,558,000)	$ 909,000
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Senior unsecured notes			7,729,405,000				8,548,055,000
Secured debt			$ 2,963,186,000				3,509,142,000
Carrying values of properties securing the debt							6,285,511,000
Debt instrument maturity date		Oct. 31, 2018
Senior Unsecured Notes [Member]
Principal payments due on debt obligations
2016	[7],[8]						400,000,000
2017	[7],[8]						450,000,000
2018	[7],[8]						450,000,000
2019	[1],[2],[7],[8]						1,280,649,000
2020	[3],[7],[8]						666,779,000
Thereafter	[4],[5],[6],[7],[8]						5,398,330,000
Totals	[7],[8]						8,645,758,000
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Notes, annual stated interest rates, Minimum		1.40%
Notes, annual stated interest rates, Maximum		6.50%
Secured Debt [Member]
Principal payments due on debt obligations
2016	[7],[9]						547,325,000
2017	[7],[9]						476,661,000
2018	[7],[9]						650,763,000
2019	[1],[2],[7],[9]						380,588,000
2020	[3],[7],[9]						173,833,000
Thereafter	[4],[5],[6],[7],[9]						1,249,037,000
Totals	[7],[9]						3,478,207,000
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Notes, annual stated interest rates, Minimum		1.00%
Notes, annual stated interest rates, Maximum		7.98%
Canadian Denominated Unsecured Term Loan [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		1.80%
Senior unsecured notes					CAD 250,000		180,649,000
Interest Rate margin		0.975%
Debt Instrument, Issuance Date		Jul. 25, 2014
Debt instrument maturity date		Oct. 31, 2018
Unsecured Line of Credit Term Loan [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		1.40%
Senior unsecured notes							500,000,000
Interest Rate margin		0.975%
Debt Instrument, Issuance Date		Jul. 25, 2014
Debt instrument maturity date		Oct. 31, 2018
3.35% Canadian Senior Secured Note Due 2020 [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		3.35%
Senior unsecured notes					CAD 300,000		216,779,000
Debt Instrument, Issuance Date		Nov. 25, 2015
Debt instrument maturity date		Jan. 01, 2020
Uk Debt Due 2028 [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.80%
Senior unsecured notes						£ 550,000	811,030,000
Debt Instrument, Issuance Date		Nov. 20, 2013
Debt instrument maturity date		Jan. 01, 2028
Uk Debt Due 2034 [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.50%
Senior unsecured notes						£ 500,000	737,300,000
Debt Instrument, Issuance Date		Nov. 25, 2014
Debt instrument maturity date		Jan. 01, 2034
Four Percent Senior Unsecured Notes Due 2025 [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.00%
Senior unsecured notes							750,000,000
Debt instrument maturity date		Jan. 01, 2025
Senior Unsecured Notes Additional Issued Amount							500,000,000
Four Percent Senior Unsecured Notes Due 2025 [Member] | Debt Instrument Redemption Period One [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Debt Instrument, Issuance Date		May 01, 2015
Four Percent Senior Unsecured Notes Due 2025 [Member] | Debt Instrument Redemption Period Two [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Debt Instrument, Issuance Date		Oct. 01, 2015
3.00% Senior Unsecured Convertible Notes due 2029
Senior Unsecured Convertible Notes Issued [Abstract]
Convertible Debt Issued Amount							$ 494,403,000
Conversion rate		19.5064
Conversion price | $ / shares							$ 51.27
Equity component of convertible debt, reacquire		$ 29,925,000
Repurchase Percentage					100.00%	100.00%	100.00%
Debt Conversion Amount		$ 215,965,000
Conversion Share Amount | shares		366,211
Gains (Losses) on Extinguishment of Debt		$ 5,881,000
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		3.00%
Debt Instrument, Issuance Date		Dec. 31, 2010
3.00% Senior Unsecured Convertible Notes due 2029 | Debt Instrument Redemption Period One [Member]
Senior Unsecured Convertible Notes Issued [Abstract]
Repurchase Date		Dec. 01, 2019
3.00% Senior Unsecured Convertible Notes due 2029 | Debt Instrument Redemption Period Two [Member]
Senior Unsecured Convertible Notes Issued [Abstract]
Repurchase Date		Dec. 01, 2024

[1] On July 25, 2014, we refinanced the funding on a $250,000,000 Canadian-denominated unsecured term credit facility (approximately $180,649,000 based on the Canadian/U.S. Dollar exchange rate on December 31, 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 (1.8% at December 31, 2015).

[2] 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 (1.4% at December 31, 2015).

[3] In November 2015, we issued $300,000,000 of Canadian-denominated 3.35% senior unsecured notes due 2020 (approximately $216,779,000 based on the Canadian/U.S. Dollar exchange rate on December 31, 2015).

[4] In May 2015, we issued $750,000,000 of 4.0% senior unsecured notes due 2025. In October 2015, we issued an additional $500,000,000 of these notes under a re-opening of the offer.

[5] On November 20, 2013, we completed funding on £550,000,000 (approximately $811,030,000 based on the Sterling/U.S. Dollar exchange rate on December 31, 2015) of 4.8% senior unsecured notes due 2028.

[6] On November 25, 2014, we completed funding on £500,000,000 (approximately $737,300,000 based on the Sterling/U.S. Dollar exchange rate on December 31, 2015) of 4.5% senior unsecured notes due 2034.

[7] Amounts represent principal amounts due and do not include unamortized premiums/discounts, debt issuance costs, or other fair value adjustments as reflected on the consolidated balance sheet.

[8] Annual interest rates range from 1.4% to 6.5%.

[9] Annual interest rates range from 1.0% to 7.98%. Carrying value of the properties securing the debt totaled $6,285,511,000 at December 31, 2015.