Mortgage Servicing Rights - Additional Information (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
Transfers And Servicing [Abstract]
Mortgage servicing rights sold	$ 11,935,000	$ 11,935,000
Mortgage loans serviced	$ 1,085,465,000	$ 1,085,465,000	$ 3,217,580,000
Mortgage loans serviced related to bulk sale of mortgage servicing rights			$ 0	$ 0

Amount conveyed to unrelated parties of contract to contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746

Name:	us-gaap_ServicingAssetAtAmortizedValueDisposals
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration