Reclassifications of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains		$ (12,942)	$ 38,100	$ (25,873)	$ 25,074
Change in allowance for credit losses on available for sale securities		(246)	(17,021)	(2,218)	14,354
Income taxes		102,281	53,356	161,148	(51,364)
Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications		(10,400)	(5,600)	(22,200)	(16,400)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains	[1]	(12,900)	9,900	(25,900)	(35,200)
Change in allowance for credit losses on available for sale securities		(200)	(17,000)	(2,200)	14,400
Income taxes		$ 2,700	$ 1,500	$ 5,900	$ 4,400

[1] For the three months ended June 30, 2020, excludes: (i) the $16.3 million Wilbert Remeasurement Gain; and (ii) $44.5 million of impairment charges from a write-down of SORC oil field assets. For the six months ended June 30, 2020, excludes: (i) the $16.3 million Wilbert Remeasurement Gain; (ii) a $7.1 million realized loss as a result of an early redemption of debt (Note 8 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2020 Form 10-K for further information); (iii) $74.4 million of impairment charges from a write-down of SORC oil field assets; and (iv) a $5.0 million realized gain resulting from a reduction of certain PCT contingent consideration liabilities (see Note 3(e) of this Form 10-Q for additional information).