Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $)	Mar. 31, 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	$ 513,544,000		$ 715,553,000
Mortgage and asset-backed securities, amortized cost or cost	4,576,000,000	[1]
Debt securities with maturity dates, amortized cost or cost:
One year or less	585,200,000
Over one through five years	3,254,500,000
Over five through ten years	2,764,400,000
Over ten years	3,183,400,000
Debt securities, amortized cost	14,363,443,000		14,364,430,000
Equity securities, cost	2,573,160,000		2,366,035,000
Amortized Cost or Cost	17,450,300,000		17,446,100,000
Short-term investments due in one year or less, fair value	513,600,000
Mortgage and asset-backed securities, fair value	4,631,500,000	[1]
Debt securities with maturity dates, fair value:
One year or less	588,800,000
Over one through five years	3,306,600,000
Over five through ten years	2,858,500,000
Over ten years	3,284,800,000
Total debt securities, fair value	14,670,207,000		14,598,641,000
Equity securities, fair value	3,053,543,000		2,815,484,000
Fair Value	$ 18,237,300,000		$ 18,129,700,000

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