Organization and Operation (Details) ft² in Millions	Dec. 31, 2017 ft² state property
Organization and Operation
Properties owned \| property	5,172
Number of U.S. states where properties are owned \| state	49
Leasable square feet \| ft²	89.6

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

No definition available.

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