Senior Unsecured Notes and Secured Debt (Details) £ in Thousands, CAD in Thousands, $ in Thousands		3 Months Ended
		Mar. 31, 2016 CAD	Mar. 31, 2016 GBP (£)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 01, 2015 USD ($)
Principal payments due on debt obligations
2016				$ 434,322
2017				940,995
2018				1,081,533
2019	[1],[2]			1,678,515
2020	[3]			863,875
Thereafter	[4],[5],[6],[7]			7,423,341
Totals				12,422,581
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				8,828,053	$ 8,548,055
Secured debt				3,515,053	$ 3,509,142
Debt instrument maturity date		Oct. 31, 2018
Senior Unsecured Notes [Member]
Principal payments due on debt obligations
2016	[8],[9]			0
2017	[8],[9]			450,000
2018	[8],[9]			450,000
2019	[1],[2],[8],[9]			1,292,767
2020	[3],[8],[9]			681,321
Thereafter	[4],[5],[6],[7],[8],[9]			6,060,005
Totals	[8],[9]			8,934,093
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Notes, annual stated interest rates, Minimum		1.41%
Notes, annual stated interest rates, Maximum		6.50%
Secured Debt [Member]
Principal payments due on debt obligations
2016	[8],[10]			434,322
2017	[8],[10]			490,995
2018	[8],[10]			631,533
2019	[1],[2],[8],[10]			385,748
2020	[3],[8],[10]			182,554
Thereafter	[4],[5],[6],[7],[8],[10]			1,363,336
Totals	[8],[10]			3,488,488
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				6,226,111
4.00% Senior Unsecured Notes Due 2025 [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes issued amount				750,000
Debt Instrument, Issuance Date		May 01, 2015
Senior Unsecured Notes Additional Issued Amount						$ 500,000
Canadian Denominated Unsecured Term Loan [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes		CAD 250,000		$ 192,767
Interest Rate margin		1.85%
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%
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	$ 719,050
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	$ 790,955
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
Interest Rate margin		1.41%
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%
4.25% Senior Unsecured Notes Due 2026
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes issued amount				700,000
Debt Instrument, Issuance Date		Mar. 01, 2016
3.35% senior unsecured note due 2020
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes issued amount		CAD 300,000		$ 231,321
Debt Instrument, Issuance Date		Nov. 01, 2015

[1] (4) On July 25, 2014, we refinanced the funding on a $250,000,000 Canadian-denominated unsecured term credit facility (approximately $192,767,000 based on the Canadian/U.S. Dollar exchange rate on March 31, 2016). 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 (1.85% at March 31, 2016).

[2] (5) 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 an additional year at our discretion) and bears interest at LIBOR plus 97.5 basis points (1.41% at March 31, 2016).

[3] (6) In November 2015, one of our wholly-owned subsidiaries issued and we guaranteed $300,000,000 of Canadian-denominated 3.35% senior unsecured notes due 2020 (approximately $231,321,000 based on the Canadian/U.S. Dollar exchange rate on March 31, 2016).

[4] (10) In March 2016, we issued $700,000,000 of 4.25% senior unsecured notes due 2026.

[5] (7) On November 20, 2013, we completed the sale of £550,000,000 (approximately $790,955,000 based on the Sterling/U.S. Dollar exchange rate in effect on March 31, 2016) of 4.8% senior unsecured notes due 2028.

[6] (8) On November 25, 2014, we completed the sale of £500,000,000 (approximately $719,050,000 based on the Sterling/U.S. Dollar exchange rate in effect on March 31, 2016) of 4.5% senior unsecured notes due 2034.

[7] (9) 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.

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

[9] (2) Annual interest rates range from 1.41% to 6.5%.

[10] (3) Annual interest rates range from 1.0% to 7.98%. Carrying value of the properties securing the debt totaled $6,226,111,000 at March 31, 2016.