Senior Unsecured Notes and Secured Debt (Details) £ in Thousands, CAD in Thousands, $ in Thousands		6 Months Ended
		Jun. 30, 2016 CAD	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 01, 2015 USD ($)
Principal payments due on debt obligations
2016				$ 304,577
2017				1,006,382
2018				1,081,424
2019				990,735
2020	[1]			856,929
Thereafter	[2],[3],[4],[5],[6],[7]			7,998,743
Totals				12,238,790
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				8,711,790	$ 8,548,055
Secured debt				3,442,178	$ 3,509,142
Interest Rate margin		0.90%
Debt instrument maturity date		May 13, 2020
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	[8],[9]			605,000
2020	[1],[8],[9]			680,645
Thereafter	[2],[3],[4],[5],[6],[7],[8],[9]			6,631,879
Totals	[8],[9]			8,817,524
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Notes, annual stated interest rates, Minimum		1.15%
Notes, annual stated interest rates, Maximum		6.50%
Secured Debt [Member]
Principal payments due on debt obligations
2016	[8],[10]			304,577
2017	[8],[10]			556,382
2018	[8],[10]			631,424
2019	[8],[10]			385,735
2020	[1],[8],[10]			176,284
Thereafter	[2],[3],[4],[5],[6],[7],[8],[10]			1,366,864
Totals	[8],[10]			3,421,266
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Notes, annual stated interest rates, Minimum		0.91%
Notes, annual stated interest rates, Maximum		7.98%
Carrying values of properties securing the debt				5,972,202
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		$ 192,204
Interest Rate margin		1.84%
Debt Instrument, Issuance Date		May 13, 2016
Debt instrument maturity date		May 13, 2021
Canadian Denominated Unsecured Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest Rate margin		0.95%
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	$ 661,750
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	$ 727,925
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		1.40%
Debt Instrument, Issuance Date		May 13, 2016
Debt instrument maturity date		May 13, 2021
Unsecured term credit facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest Rate margin		0.95%
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		$ 230,645
Debt Instrument, Issuance Date		Nov. 01, 2015

[1] (4) 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 $230,645,000 based on the Canadian/U.S. Dollar exchange rate on June 30, 2016).

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

[3] (5) On May 13, 2016, we refinanced the funding on a $250,000,000 Canadian-denominated unsecured term credit facility (approximately $192,204,000 based on the Canadian/U.S. Dollar exchange rate on June 30, 2016). The loan matures on May 13, 2021 and bears interest at the Canadian Dealer Offered Rate plus 95 basis points (1.84% at June 30, 2016).

[4] (6) On May 13, 2016, we refinanced the funding on a $500,000,000 unsecured term credit facility. The loan matures on May 13, 2021 and bears interest at LIBOR plus 95 basis points (1.40% at June 30, 2016).

[5] (7) On November 20, 2013, we completed the sale of £550,000,000 (approximately $727,925,000 based on the Sterling/U.S. Dollar exchange rate in effect on June 30, 2016) of 4.8% senior unsecured notes due 2028.

[6] (8) On November 25, 2014, we completed the sale of £500,000,000 (approximately $661,750,000 based on the Sterling/U.S. Dollar exchange rate in effect on June 30, 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 1.15% to 6.5%.

[10] (3) Annual interest rates range from 0.91% to 7.98%. Carrying value of the properties securing the debt totaled $5,972,202,000 at June 30, 2016.