Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 968,500
Mortgage and asset-backed securities, amortized cost	[1]	5,211,500
Debt securities with maturity dates, amortized cost:
One year or less		513,800
Over one through five years		3,565,800
Over five through ten years		2,669,200
Over ten years		2,052,000
Debt securities, amortized cost		14,012,407	$ 11,895,850
Short-term investments due in one year or less, fair value		968,547	893,776
Mortgage and asset-backed securities, fair value	[1]	5,314,400
Debt securities with maturity dates, fair value:
One year or less		516,000
Over one through five years		3,638,900
Over five through ten years		2,802,200
Over ten years		2,216,800
Total debt securities, fair value		$ 14,488,289	$ 11,823,968

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