Summary of Significant Accounting Policies - Loans Receivable (Details)	12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]
Threshold period past due for write-off of financing receivable	90 days

Threshold period past due for financing receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_ThresholdPeriodPastDueForWriteoffOfFinancingReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration