Effect of Adopting ASU 2010-26 in Condensed Consolidated Statements of Comprehensive Income (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Underwriting, acquisition and insurance expenses	$ 274	$ 262
Net income	86	61
Net income per share:
Basic	$ 0.53	$ 0.38
Diluted	$ 0.53	$ 0.38
Accounting Standards Update 2010-26
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Underwriting, acquisition and insurance expenses	274
Net income	86
Net income per share:
Basic	$ 0.53
Diluted	$ 0.53
Accounting Standards Update 2010-26 | As Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Underwriting, acquisition and insurance expenses		261
Net income		62
Net income per share:
Basic		$ 0.38
Diluted		$ 0.38
Accounting Standards Update 2010-26 | As Adjusted
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Underwriting, acquisition and insurance expenses		262
Net income		61
Net income per share:
Basic		$ 0.38
Diluted		$ 0.38
Accounting Standards Update 2010-26 | Difference
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Underwriting, acquisition and insurance expenses		1
Net income		$ (1)
Net income per share:
Basic		$ 0.00
Diluted		$ 0.00