Significant Accounting Policies - Accounting Standards Update (Details) $ in Billions	3 Months Ended
Mar. 31, 2018 USD ($)
Scenario, Forecast \| 1515 Broadway \| Accounting Standards Update 2017-05
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gain on disposal	$ 0.6

Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2443-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2941-110230

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