Revenue Recognition - Additional Information (Detail) - ASC 606 [Member]	9 Months Ended
Sep. 30, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of insurance agency income derived from direct-bill customers	75.00%
Percentage of insurance agency income derived from agency-billed customer	25.00%
Percentage of trust fee income recognized	85.00%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration