Investments - Schedule of Equity Securities and Trading Debt Securities Net Unrealized Investment Gains and Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Realized Or Unrealized Gain Loss On Trading Securities [Abstract]
Net unrealized investment gains (losses)	$ (43,009)	$ 27,192	$ 11,816

Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

No definition available.

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