Mortgage Servicing Rights - Additional Information (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Transfers And Servicing [Abstract]
Mortgage servicing rights sold	$ 11,686,000	$ 34,118,000
Mortgage loans serviced	1,086,465,000	3,370,395,000
Mortgage loans serviced related to bulk sale of mortgage servicing rights	$ 0	$ 3,332,903,000

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