Senior Unsecured Notes and Secured Debt (Details) £ in Thousands, CAD in Thousands, $ in Thousands		6 Months Ended
		Jun. 30, 2017 CAD	Jun. 30, 2017 GBP (£)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 01, 2015 USD ($)
Principal payments due on debt obligations
2017				$ 173,105
2018				855,834
2019				1,085,977
2020	[1]			837,674
2021	[2],[3]			1,356,926
Thereafter	[4],[5],[6],[7]			6,708,312
Totals				11,017,828
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				8,250,940	$ 8,161,619
Secured debt				2,670,914	$ 3,477,699
Maturity Date		May 13, 2021
Senior Unsecured Notes [Member]
Principal payments due on debt obligations
2017	[8],[9]			0
2018	[8],[9]			450,000
2019	[8],[9]			605,000
2020	[1],[8],[9]			681,089
2021	[2],[3],[8],[9]			1,142,574
Thereafter	[4],[5],[6],[7],[8],[9]			5,464,476
Totals	[8],[9]			$ 8,343,139
Senior Unsecured Notes [Member] | Maximum [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest rate		6.50%	6.50%	6.50%
Senior Unsecured Notes [Member] | Minimum [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest rate		1.80%	1.80%	1.80%
Secured Debt [Member]
Principal payments due on debt obligations
2017	[8],[10]			$ 173,105
2018	[8],[10]			405,834
2019	[8],[10]			480,977
2020	[1],[8],[10]			156,585
2021	[2],[3],[8],[10]			214,352
Thereafter	[4],[5],[6],[7],[8],[10]			1,243,836
Totals	[8],[10]			2,674,689
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Carrying values of properties securing the debt				$ 5,624,262
Secured Debt [Member] | Maximum [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest rate		7.98%	7.98%	7.98%
Secured Debt [Member] | Minimum [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Interest rate		1.42%	1.42%	1.42%
3.35% senior unsecured note due 2020
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes issued amount		CAD 300,000		$ 231,089
Debt Instrument, Issuance Date		Nov. 01, 2015
Canadian Denominated Unsecured Term Loan [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes		CAD 250,000		192,574
Debt Instrument Basis Spread On Variable Rate1		1.79%
Debt Instrument, Issuance Date		May 13, 2016
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]
Debt Instrument Basis Spread On Variable Rate1		0.95%
Unsecured term credit facility [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Senior unsecured notes				$ 500,000
Debt Instrument Basis Spread On Variable Rate1		2.08%
Debt Instrument, Issuance Date		May 13, 2016
Maturity Date		May 13, 2021
Unsecured term credit facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Senior Unsecured Notes and Secured Debt (Textual) [Abstract]
Debt Instrument Basis Spread On Variable Rate1		0.95%
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	$ 714,725
Debt Instrument, Issuance Date		Nov. 20, 2013
Maturity Date		Nov. 20, 2028
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	$ 649,750
Debt Instrument, Issuance Date		Nov. 25, 2014
Maturity Date		Nov. 24, 2034
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
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
Maturity Date		Mar. 01, 2026

[1] 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 $231,089,000 based on the Canadian/U.S. Dollar exchange rate on June 30, 2017).

[2] On May 13, 2016, we refinanced the funding on a $250,000,000 Canadian-denominated unsecured term credit facility (approximately $192,574,000 based on the Canadian/U.S. Dollar exchange rate on June 30, 2017). The loan matures on May 13, 2021 and bears interest at the Canadian Dealer Offered Rate plus 95 basis points (1.79% at June 30, 2017).

[3] 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 (2.08% at June 30, 2017).

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

[5] 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.

[6] On November 20, 2013, we completed the sale of £550,000,000 (approximately $714,725,000 based on the Sterling/U.S. Dollar exchange rate in effect on June 30, 2017) of 4.8% senior unsecured notes due 2028.

[7] On November 25, 2014, we completed the sale of £500,000,000 (approximately $649,750,000 based on the Sterling/U.S. Dollar exchange rate in effect on June 30, 2017) of 4.5% senior unsecured notes due 2034.

[8] 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] Annual interest rates range from 1.8% to 6.5%.

[10] Annual interest rates range from 1.42% to 7.98%. Carrying value of the properties securing the debt totaled $5,624,262,000 at June 30, 2017.