Segments of Business - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013 Segment	Jun. 30, 2012 Segment	Jun. 30, 2013 Segment	Jun. 30, 2012 Segment	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of reportable segments	2	2	2	2
Identifiable assets	$ 22,890,080,000		$ 22,890,080,000		$ 22,807,967,000
Stockholders' equity	6,498,372,000		6,498,372,000		6,403,787,000
Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	16,300,000,000		16,300,000,000
Cash and invested assets	14,100,000,000		14,100,000,000
Stockholders' equity	4,200,000,000		4,200,000,000
Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	5,900,000,000		5,900,000,000
Cash and invested assets	4,300,000,000		4,300,000,000
Stockholders' equity	2,400,000,000		2,400,000,000
Corporate activities
Segment Reporting Information [Line Items]
Identifiable assets	700,000,000		700,000,000
Cash and invested assets	400,000,000		400,000,000
Stockholders' equity	$ (100,000,000)		$ (100,000,000)