Realized and Net Change in Unrealized Gains (Losses) in Other Investments (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Gain (Loss) on Securities [Line Items]
Total Investment Income	$ 597,947	$ 1,176,392	$ 1,746,626	$ 2,228,040
Other Investments
Gain (Loss) on Securities [Line Items]
Realized Gains	21,491	16,207	45,727	16,319
Net Change in Unrealized Gains (Losses)	(3,933)	49,643	6,789	45,411
Total Investment Income	$ 17,558	$ 65,850	$ 52,516	$ 61,730

Amount of realized and unrealized gain (loss) on investment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

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