Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 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		$ 493,614	$ 365,810
Mortgage and asset-backed securities, amortized cost or cost	[1]	3,892,100
Debt securities with maturity dates, amortized cost or cost:
One year or less		416,200
Over one through five years		2,851,500
Over five through ten years		3,079,900
Over ten years		3,145,600
Debt securities, amortized cost		13,385,266	13,529,923
Equity securities, cost		2,637,787	2,740,984
Amortized Cost or Cost		16,516,700	16,636,700
Short-term investments due in one year or less, fair value		493,600
Mortgage and asset-backed securities, fair value	[1]	3,865,800
Debt securities with maturity dates, fair value:
One year or less		418,100
Over one through five years		2,895,300
Over five through ten years		3,176,600
Over ten years		3,262,400
Total debt securities, fair value		13,618,212	13,605,963
Equity securities, fair value		2,889,055	3,005,908
Fair Value		$ 17,000,900	$ 16,977,700

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