Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $)	Mar. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Short-term investments due in one year or less, amortized cost or cost	$ 757,109,000		$ 366,044,000
Mortgage and asset-backed securities, amortized cost or cost	2,591,700,000	[1]
Debt securities with maturity dates, amortized cost or cost:
One year or less	619,200,000
Over one through five years	4,223,200,000
Over five through ten years	3,661,300,000
Over ten years	3,660,400,000
Total debt securities, amortized cost or cost	14,755,756,000		15,593,278,000
Equity securities, amortized cost or cost	1,852,640,000		1,436,540,000
Amortized cost or cost	17,365,500,000		17,395,800,000
Short-term investments due in one year or less, fair value	757,100,000
Mortgage and asset-backed securities, fair value	2,652,900,000	[1]
Debt securities with maturity dates, fair value:
One year or less	621,500,000
Over one through five years	4,297,300,000
Over five through ten years	3,784,000,000
Over ten years	3,754,800,000
Total debt securities, fair value	15,110,495,000		15,999,538,000
Equity securities, fair value	2,004,896,000		1,424,014,000
Fair Value	$ 17,872,500,000		$ 17,789,500,000

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