Summary of Significant Accounting Principles - Allowance for Credit Losses (Details)	12 Months Ended
Dec. 31, 2017
Home equity lines of credit \| Second Mortgage
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity	100.00%

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Name:	us-gaap_FairValueInputsLossSeverity
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration