Commitments and Contingencies - Additional Information (Detail) - Mortgage Partnership Finance Program [Member] - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Commitments	$ 12,315,000	$ 1,882,000
Other commitments	$ 0	$ 0

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

No definition available.

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant