Fair Value of Financial Instruments - Additional Information (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unrealized gain (losses) relating to assets still held and included in net investment income	$ 127,219	$ 69,005
Price Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity volatility (as percent)	0.461		0.113

Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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