Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 379,000
Mortgage and asset-backed securities, amortized cost	[1]	5,648,400
Debt securities with maturity dates, amortized cost:
One year or less		592,700
Over one through five years		3,774,900
Over five through ten years		2,856,900
Over ten years		2,458,200
Debt securities, amortized cost		15,331,053	$ 14,898,522
Short-term investments due in one year or less, fair value		379,009	714,208
Mortgage and asset-backed securities, fair value	[1]	5,760,300
Debt securities with maturity dates, fair value:
One year or less		599,900
Over one through five years		3,888,200
Over five through ten years		2,981,200
Over ten years		2,635,300
Total debt securities, fair value		$ 15,864,867	$ 15,618,470

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