Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Short-term investments due in one year or less, amortized cost or cost		$ 778,410	$ 365,810
Mortgage and asset-backed securities, amortized cost or cost	[1]	3,232,300
Debt securities with maturity dates, amortized cost or cost:
One year or less		428,600
Over one through five years		3,048,500
Over five through ten years		3,229,100
Over ten years		2,988,600
Debt securities, amortized cost		12,927,103	13,529,923
Equity securities, cost		2,816,572	2,740,984
Amortized Cost or Cost		16,522,100	16,636,700
Short-term investments due in one year or less, fair value		778,400
Mortgage and asset-backed securities, fair value	[1]	3,225,100
Debt securities with maturity dates, fair value:
One year or less		430,800
Over one through five years		3,074,500
Over five through ten years		3,250,700
Over ten years		3,002,100
Total debt securities, fair value		12,983,213	13,605,963
Equity securities, fair value		3,109,523	3,005,908
Fair Value		$ 16,871,100	$ 16,977,700

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