Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 1,119,700
Mortgage and asset-backed securities, amortized cost	[1]	5,301,800
Debt securities with maturity dates, amortized cost:
One year or less		423,100
Over one through five years		3,431,600
Over five through ten years		3,172,400
Over ten years		2,066,600
Debt securities, amortized cost		14,395,510	$ 13,798,576
Short-term investments due in one year or less, fair value		1,119,678	914,776
Mortgage and asset-backed securities, fair value	[1]	5,350,800
Debt securities with maturity dates, fair value:
One year or less		427,200
Over one through five years		3,568,900
Over five through ten years		3,363,300
Over ten years		2,239,800
Total debt securities, fair value		$ 14,950,006	$ 14,211,745

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