Borrowings Under Lines of Credit Arrangements and Related Items (Details Textuals) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) Number	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Line Of Credit Facility [Line Items]
Borrowings under unsecured line of credit arrangement	[1]	$ 835,000	$ 0	$ 130,000
Unsecured line of credit		$ 2,500,000
Number of banks in consortium | Number		28
Applicable margin over index, based on certain debt ratings		0.925%
Facility Fee as Percentage of commitment fee		0.15%
Maturity Date		Oct. 31, 2018
Available in Alternate Currencies		$ 500,000
Line of Credit Interest Rate, ending		1.347%
Term Loan		$ 500,000
Canadian Denominated Term Loan		250,000
Accordion Feature [Member]
Line Of Credit Facility [Line Items]
Term Loan		1,000,000
Canadian Denominated Term Loan		250,000
Financial Stand by Letter Of Credit [Member]
Line Of Credit Facility [Line Items]
Borrowings under unsecured line of credit arrangement		$ 54,925

[1] As of December 31, 2015, letters of credit in the aggregate amount of $54,925,000 have been issued which reduce the available borrowing capacity on the primary unsecured credit facility.