Realized and Net Change in Unrealized Gains (Losses) in Other Investments (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gain (Loss) on Securities [Line Items]
Total Investment Income	$ 3,473,813	$ 2,903,659	$ 4,144,712
Other Investments
Gain (Loss) on Securities [Line Items]
Realized Gains	46,248	56,381	4,886
Net Change in Unrealized Gains	21,450	20,335	14,324
Total Investment Income	$ 67,698	$ 76,716	$ 19,210

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
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Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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