Description of Business (Details)	Dec. 31, 2017 item
Description of Business
Number of states in which the entity operates	50
Number of countries in which the entity operates	44

The number of countries in which the entity operates as of balance sheet date.

No definition available.

Name:	us-gaap_NumberOfCountriesInWhichEntityOperates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant