Summary of Significant Accounting Policies - Investments in Affiliates and Other Entities (Detail) - Capital for infrastructure projects $ in Millions	Sep. 30, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Other commitment	$ 80.0
Infrastructure investors partnership
Summary Of Significant Accounting Policies [Line Items]
Other commitment	$ 1,000.0

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