Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair value of Cash and Cash Equivalents by Balance Sheet Grouping

Cash and cash equivalents consist of the following:

	December 31, 2017			December 31, 2016
	Cost		Fair Value	Cost	Fair Value
Cash and cash equivalents:
Cash and overnight deposits	$	383,021	383,021	406,787	406,787
Corporate commercial paper	635,345		635,919	507,777	507,889
Time deposits	32,733		32,733	59,871	59,871
Total cash and cash equivalents	1,051,099		1,051,673	974,435	974,547