Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 8,456,239	$ 8,685,914	$ 8,776,734	$ 7,692,710
Other comprehensive income (loss), net of tax:
Ending Balance	8,596,395	8,828,658	8,596,395	8,828,658
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			321,000	(61,600)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications			212,200	442,200
Reclassifications from accumulated other comprehensive income	(6,300)	(4,900)	(22,800)	(10,200)
Total			189,400	432,000
Ending Balance	510,400	370,400	510,400	370,400
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(121,800)	(124,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications			13,200	(8,100)
Reclassifications from accumulated other comprehensive income			0	0
Total			13,200	(8,100)
Ending Balance	(108,600)	(132,800)	(108,600)	(132,800)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(27,900)	(15,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications			1,400	1,400
Reclassifications from accumulated other comprehensive income			0	0
Total			1,400	1,400
Ending Balance	(26,500)	(14,300)	(26,500)	(14,300)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	291,993	151,964	171,350	(202,003)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications			226,800	435,500
Reclassifications from accumulated other comprehensive income			(22,800)	(10,200)
Total			204,000	425,300
Ending Balance	$ 375,310	$ 223,266	$ 375,310	$ 223,266