Variable Interest Entities - Additional Information (Detail)	6 Months Ended
Jun. 30, 2018 Trust
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Number of statutory business trusts	14
Percentage of equity shares of each trust owned by the company	100.00%

Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=SL6759159-111685

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