Borrowings Under Lines of Credit Arrangements and Related Items (Details Textuals) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Number
Line Of Credit Facility [Line Items]
Unsecured revolving credit facility	$ 3,000,000
Number of banks in consortium | Number	29
Available in alternate currencies	$ 1,000,000
Annual facility fee for each bank based on commitment amount	0.15%
Applicable margin over index, based on certain debt ratings	0.90%
Debt instrument maturity date	May 13, 2021
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
Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Agent banks prime rate of interest	2.46%
Debt instrument maturity date	May 13, 2020
Letter of credit that reduces the borrowing capacity	$ 22,365