Commitments and Contingencies - Additional Information (Detail) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Contractual commitments related to its limited partnership investments (up to)	$ 114.9
Contractual commitments period (not to exceed)	13 years

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

No definition available.

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