Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Available-for-sale [Line Items]
Short-term investments due in one year or less, amortized cost or cost		$ 472,800
Mortgage and asset-backed securities, amortized cost or cost	[1]	3,596,300
Debt securities with maturity dates, amortized cost or cost:
One year or less		348,200
Over one through five years		3,042,600
Over five through ten years		2,997,000
Over ten years		2,221,900
Debt securities, amortized cost		12,206,035	$ 12,536,772
Short-term investments due in one year or less, fair value		472,830	578,054
Mortgage and asset-backed securities, fair value	[1]	3,566,100
Debt securities with maturity dates, fair value:
One year or less		348,200
Over one through five years		3,024,800
Over five through ten years		2,973,600
Over ten years		2,243,200
Debt securities, fair value		$ 12,155,866	$ 12,721,399

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