Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Hedge derivative expiration period	1 month
Proceeds from the sales and maturities of securities available-for-sale	$ 308.3	$ 170.1

The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period.

No definition available.

Name:	us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration