Senior Unsecured Notes and Secured Debt (Annual Principal Payments Due on Debt Obligations) (Details) £ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Principal payments due on debt obligations
2019	$ 1,108,899,000
2020	815,777,000
2021	819,124,000
2022	880,418,000
2023	2,108,696,000
Thereafter	6,452,781,000
Totals	12,185,695,000
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Senior unsecured notes	9,603,299,000			$ 8,331,722,000
Senior Unsecured Notes
Principal payments due on debt obligations
2019	600,000,000
2020	677,489,000
2021	450,000,000
2022	600,000,000
2023	1,783,325,000
Thereafter	5,589,170,000
Totals	$ 9,699,984,000			8,417,447,000	$ 8,260,038,000	$ 8,645,758,000
Senior Unsecured Notes | Maximum
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	6.50%	6.50%	6.50%
Senior Unsecured Notes | Minimum
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	3.05%	3.05%	3.05%
Secured Debt
Principal payments due on debt obligations
2019	$ 508,899,000
2020	138,288,000
2021	369,124,000
2022	280,418,000
2023	325,371,000
Thereafter	863,611,000
Totals	2,485,711,000			$ 2,618,408,000	$ 3,465,066,000	$ 3,478,207,000
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Carrying values of properties securing the debt	$ 5,347,428,000
Secured Debt | Maximum
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	12.00%	12.00%	12.00%
Secured Debt | Minimum
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	1.69%	1.69%	1.69%
3.35% senior unsecured note due 2020
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	3.35%	3.35%	3.35%
Guarantees	$ 219,989,000		$ 300,000
Canadian Denominated Unsecured Term Loan
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Senior unsecured notes	183,325,000		$ 250,000
Unsecured term credit facility
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Senior unsecured notes	$ 500,000,000
UK Debt Due 2028
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	4.80%	4.80%	4.80%
Senior unsecured notes issued amount	$ 701,470,000	£ 550,000
UK Debt Due 2034
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	4.50%	4.50%	4.50%
Senior unsecured notes issued amount	$ 637,700,000	£ 500,000
London Interbank Offered Rate (LIBOR) | Canadian Denominated Unsecured Term Loan
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate margin	0.90%
London Interbank Offered Rate (LIBOR) | Unsecured term credit facility
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate margin	0.90%
Canadian Denominated Unsecured Term Loan | Canadian Denominated Unsecured Term Loan
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	3.15%	3.15%	3.15%
Canadian Denominated Unsecured Term Loan | Unsecured term credit facility
Senior Unsecured Notes and Secured Debt Textual [Abstract]
Interest rate	3.37%	3.37%	3.37%