Fair Value Measurements - Schedule of Changes in Fair Value Included in Earnings of Financial Instruments for which Fair Value Option has been Elected (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Mortgage Banking [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Income from mortgage banking activities	$ 3,929	$ (2,439)	$ 3,092	$ (2,439)

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

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Period Type:	duration