Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($)
Other Income (Loss) [Member]
Significant Accounting Policies [Line Items]
Unrealized gains of marketable securities and investments	$ 28.7

The aggregate net change in the difference between the fair value and the carrying value, or in the comparative fair values, of marketable securities categorized as trading held at each balance sheet date, that was included in earnings for the period, which may have arisen from (a) securities classified as trading, (b) the unrealized holding gain (loss) on held-to-maturity securities transferred to the trading security category, and (c) the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) on available-for-sale securities transferred to trading securities during the period and/or any unrealized gains or losses on investments that are separately or otherwise not categorized as trading or available-for-sale.

No definition available.

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