Insurance Reserves - Letters of Credit (Details) $ in Millions	Dec. 31, 2017 USD ($)
Insurance Reserves
Total amount committed under letters of credit	$ 31.9

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant