Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 636,100
Mortgage and asset-backed securities, amortized cost	[1]	5,607,500
Debt securities with maturity dates, amortized cost:
One year or less		488,000
Over one through five years		3,407,100
Over five through ten years		3,106,900
Over ten years		2,322,400
Debt securities, amortized cost		14,931,902	$ 13,798,576
Short-term investments due in one year or less, fair value		636,130	914,776
Mortgage and asset-backed securities, fair value	[1]	5,716,800
Debt securities with maturity dates, fair value:
One year or less		492,500
Over one through five years		3,551,100
Over five through ten years		3,315,700
Over ten years		2,503,800
Total debt securities, fair value		$ 15,579,894	$ 14,211,745

[1] Mortgage and asset-backed securities by their nature do not generally have single maturity dates.