Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Short-term investments due in one year or less, amortized cost or cost		$ 601,456	$ 715,553
Mortgage and asset-backed securities, amortized cost or cost	[1]	4,319,700
Debt securities with maturity dates, amortized cost or cost:
One year or less		553,200
Over one through five years		3,131,300
Over five through ten years		2,975,600
Over ten years		3,485,500
Debt securities, amortized cost		14,465,377	14,364,430
Equity securities, cost		2,701,247	2,366,035
Amortized Cost or Cost		17,768,100	17,446,100
Short-term investments due in one year or less, fair value		601,500
Mortgage and asset-backed securities, fair value	[1]	4,323,500
Debt securities with maturity dates, fair value:
One year or less		556,600
Over one through five years		3,160,300
Over five through ten years		3,002,100
Over ten years		3,522,400
Total debt securities, fair value		14,564,927	14,598,641
Equity securities, fair value		3,143,572	2,815,484
Fair Value		$ 18,310,000	$ 18,129,700

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