Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 7,857,062	$ 8,236,487	$ 8,776,734	$ 7,692,710
Other comprehensive income (loss), net of tax:
Ending Balance	8,456,239	8,685,914	8,456,239	8,685,914
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			135,100	356,000
Reclassifications from accumulated other comprehensive income	(5,600)	(9,800)	(16,400)	(5,400)
Total			118,700	350,600
Ending Balance	439,700	289,000	439,700	289,000
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			1,100	2,300
Total			1,100	2,300
Ending Balance	(120,700)	(122,400)	(120,700)	(122,400)
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			900	1,100
Total			900	1,100
Ending Balance	(27,000)	(14,600)	(27,000)	(14,600)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(129,343)	(14,157)	171,350	(202,003)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications			137,100	359,400
Reclassifications from accumulated other comprehensive income			(16,400)	(5,400)
Total			120,700	354,000
Ending Balance	$ 291,993	$ 151,964	$ 291,993	$ 151,964