Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 714,200
Mortgage and asset-backed securities, amortized cost	[1]	5,714,900
Debt securities with maturity dates, amortized cost:
One year or less		589,800
Over one through five years		3,457,800
Over five through ten years		2,826,500
Over ten years		2,309,500
Debt securities, amortized cost		14,898,522	$ 13,798,576
Short-term investments due in one year or less, fair value		714,208	914,776
Mortgage and asset-backed securities, fair value	[1]	5,853,300
Debt securities with maturity dates, fair value:
One year or less		594,800
Over one through five years		3,605,700
Over five through ten years		3,042,400
Over ten years		2,522,300
Total debt securities, fair value		$ 15,618,470	$ 14,211,745

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