Borrowing Arrangements (Unsecured Revolving Credit Facility and Unsecured Term Loans) (Details) CAD in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2015 USD ($)	Sep. 30, 2014 CAD	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Jul. 31, 2014 CAD
Debt Instrument [Line Items]
Principal amount outstanding			$ 11,271,020,000	$ 10,888,295,000
Gain (loss) on extinguishment of debt			(14,411,000)	(5,564,000)	$ (1,201,000)
Unsecured term loan due 2020
Debt Instrument [Line Items]
Credit facility, term	5 years
Unsecured term loan due 2020 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	97.50%
Unsecured revolving credit facilities
Debt Instrument [Line Items]
Current borrowing capacity			$ 2,000,000,000
Line of credit facility optional extension period (in years)			1 year
Maximum borrowing capacity			$ 3,500,000,000.0
Principal amount outstanding			180,683,000	$ 919,099,000
Letters of credit outstanding			14,900,000
Remaining borrowing capacity			$ 1,800,000,000
Gain (loss) on extinguishment of debt					$ (1,500,000)
Unsecured revolving credit facilities | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt Instrument, description of variable rate basis			LIBOR
Basis spread on variable rate (as a percent)			1.00%
Unsecured revolving credit facilities | Unsecured term loan due 2018
Debt Instrument [Line Items]
Current borrowing capacity			$ 200,000,000
Credit facility, term			4 years
Unsecured revolving credit facilities | Unsecured term loan due 2018 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)			1.05%
Unsecured revolving credit facilities | Unsecured term loan due 2019
Debt Instrument [Line Items]
Current borrowing capacity			$ 800,000,000
Credit facility, term			5 years
Unsecured revolving credit facilities | Unsecured term loan due 2019 | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)			1.05%
Loans Payable | Unsecured term loan due 2020
Debt Instrument [Line Items]
Debt instrument, face amount	$ 900,000,000
Unsecured Debt
Debt Instrument [Line Items]
Debt instrument, face amount			$ 20,000,000
Unsecured Debt | Unsecured term loan due 2019
Debt Instrument [Line Items]
Extinguishment of debt, amount	305,000,000
Gain (loss) on extinguishment of debt	$ (1,600,000)
Unsecured Debt | Unsecured term loan due 2015
Debt Instrument [Line Items]
Debt instrument, face amount | CAD						CAD 791.0
Repayments of debt | CAD		CAD 660.0