Senior Unsecured Notes and Secured Debt (Details) £ in Thousands, CAD in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2017 CAD	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 01, 2015 USD ($)
Principal payments due on debt obligations
2018				$ 846,588
2019				1,122,458
2020				881,900
2021	[1]			1,371,512
2022	[2],[3]			834,850
Thereafter	[4],[5],[6],[7]			5,978,547
Totals				11,035,855
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Senior unsecured notes				8,331,722	$ 8,161,619
Secured debt				2,608,976	$ 3,477,699
Debt instrument maturity date		May 13, 2021
Senior Unsecured Notes [Member]
Principal payments due on debt obligations
2018	[8],[9]			450,000
2019	[8],[9]			600,000
2020	[8],[9]			697,174
2021	[1],[8],[9]			1,149,728
2022	[2],[3],[8],[9]			600,000
Thereafter	[4],[5],[6],[7],[8],[9]			4,920,545
Totals	[8],[9]			$ 8,417,447
Senior Unsecured Notes [Member] | Maximum [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		2.10%	2.10%	2.10%
Senior Unsecured Notes [Member] | Minimum [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		6.50%	6.50%	6.50%
Secured Debt [Member]
Principal payments due on debt obligations
2018	[8],[10]			$ 396,588
2019	[8],[10]			522,458
2020	[8],[10]			184,726
2021	[1],[8],[10]			221,784
2022	[2],[3],[8],[10]			234,850
Thereafter	[4],[5],[6],[7],[8],[10]			1,058,002
Totals	[8],[10]			2,618,408
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Carrying values of properties securing the debt				$ 5,475,672
Secured Debt [Member] | Maximum [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		1.69%	1.69%	1.69%
Secured Debt [Member] | Minimum [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		7.98%	7.98%	7.98%
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		$ 239,674
Debt instrument, issuance date		Nov. 01, 2015
Debt instrument maturity date		Nov. 01, 2020
Canadian Denominated Unsecured Term Loan [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		2.28%
Senior unsecured notes		CAD 250,000		199,728
Interest rate margin		0.95%
Debt instrument, issuance date		May 13, 2016
Debt instrument maturity date		May 13, 2021
Unsecured Term Credit Facility Member [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		2.41%
Senior unsecured notes				500,000
Interest rate margin		0.95%
Debt instrument, issuance date		May 13, 2016
Debt instrument maturity date		May 13, 2021
Uk Debt Due 2028 [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.80%
Senior unsecured notes			£ 550,000	744,095
Debt instrument, issuance date		Nov. 20, 2013
Debt instrument maturity date		Nov. 20, 2028
Uk Debt Due 2034 [Member]
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.50%
Senior unsecured notes			£ 500,000	676,450
Debt instrument, issuance date		Nov. 25, 2014
Debt instrument maturity date		Nov. 25, 2034
4.00% Senior Unsecured Notes Due 2025
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.00%
Senior unsecured notes				750,000
Debt instrument, issuance date		May 01, 2015
Debt instrument maturity date		May 01, 2025
Senior unsecured notes additional issued amount						$ 500,000
4.25% Senior Unsecured Notes Due 2026
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate		4.25%
Senior unsecured notes issued amount				$ 700,000
Debt instrument, issuance date		Mar. 01, 2016
Debt instrument 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 $239,674,000 based on the Canadian/U.S. Dollar exchange rate on December 31, 2017).

[2] On May 13, 2016, we refinanced the funding on a $250,000,000 Canadian-denominated unsecured term credit facility (approximately $199,728,000 based on the Canadian/U.S. Dollar exchange rate on December 31, 2017). The loan matures on May 13, 2021 and bears interest at the Canadian Dealer Offered Rate plus 95 basis points (2.28% at December 31, 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 (2.41% at December 31, 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 $744,095,000 based on the Pounds Sterling/U.S. Dollar exchange rate in effect on December 31, 2017) of 4.8% senior unsecured notes due 2028.

[7] On November 25, 2014, we completed the sale of £500,000,000 (approximately $676,450,000 based on the Pounds Sterling/U.S. Dollar exchange rate in effect on December 31, 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 consolidated balance sheet.

[9] Annual interest rates range from 2.1% to 6.5%.

[10] Annual interest rates range from 1.69% to 7.98%. Carrying value of the properties securing the debt totaled $5,475,672,000 at December 31, 2017.