Related Party Transactions (Details) $ in Thousands	12 Months Ended
Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Related party transactions
Number of leased properties \| property	3
Lease payments to related party	$ 1,495	$ 1,445	$ 1,505
SIGI
Related party transactions
Lease payments to related party	$ 849	$ 831	$ 862

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Name:	us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration