Fair Value of Financial Instruments - Additional Information (Detail)	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unrealized gain (losses) relating to assets still held and included in net investment income	$ 94,883,000	$ 50,805,000	$ 44,615,000
Transfer between Level 1 and Level 2 fair value hierarchies	$ 0	$ 0	$ 0
Price Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Equity volatility (as percent)	0.232	0.137

Value of input used to measure derivative asset.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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