Amortized Cost and Estimated Fair Value of Debt securities by Contractual Maturity (Detail) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Short-term investments due in one year or less	$ 262,400,000
Short-term investments due in one year or less	262,400,000
Mortgage and asset-backed securities	1,006,300,000
Debt securities
One year or less	244,200,000
Over one through five years	561,600,000
Over five through ten years	594,000,000
Over ten years	389,100,000
Equity securities	1,642,108,000	1,500,686,000
Fair value	4,699,700,000	4,597,900,000
Available for sale securities, amortized cost	4,375,700,000	4,352,900,000
Debt securities
One year or less	241,100,000
Over one through five years	540,700,000
Over five through ten years	569,700,000
Over ten years	383,900,000
Equity securities	1,406,500,000
Amortized Cost or Cost	4,375,700,000	4,352,900,000
Mortgage and Asset Backed Securities
Debt securities
Available for sale securities, amortized cost	971,400,000
Debt securities
Amortized Cost or Cost	$ 971,400,000