Marketable Securities and Restricted Funds Schedule of Restricted and Non-restricted Available for Sale Securities - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Unrealized Gain (Loss)	$ 0	$ 0
Available-for-sale Securities [Member] \| Maximum
Schedule of Available-for-sale Securities [Line Items]
Weighted Average Contractual Maturity Of Outstanding Securities	1 year

Amount of unrealized gain (loss) on investments in debt and equity securities classified as available-for-sale securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

Name:	us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGainLoss1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration