Commitments and Contingencies - Bonds (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($)
British Petroleum
Commitments and Contingencies
Gain on the claim settlement	$ 0.6
Surety \| Minimum
Surety
Percentage of business which has required bonds	20.00%
Surety \| Maximum
Surety
Percentage of business which has required bonds	30.00%

Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

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