Condensed Combined Statement of Income for Limited Liabilities Accounted for under Equity Method (Parenthetical) (Detail) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity Method Investments And Joint Ventures [Abstract]
Interest income earned on various advances made to LLCs	$ 1,200,000	$ 200,000	$ 834,000

Amount of interest income earned from interest bearing assets classified as other.

No definition available.

Name:	us-gaap_InterestIncomeOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration