Debt - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2008
Debt Instrument [Line Items]
Proceeds from issuance of senior notes, net		$ 990,000,000	$ 0	$ 0
Percentage of senior notes principal amount which may be redeemed at our option		100.00%
Cumulative adjustment to carrying value of senior note					103,000,000
Unamortized carrying value adjustment		64,000,000	74,000,000
Minimum level of net worth required		6,600,000,000
Maximum leverage ratios for new credit agreement		3.0
Actual net worth		8,800,000,000
Actual leverage ratio		1.1
Uncommitted incremental loan facility		250,000,000
Line of credit facility, covenant terms		The terms of the credit agreement include standard provisions related to conditions of borrowing, including a customary material adverse effect clause which could limit our ability to borrow additional funds. In addition, the credit agreement contains customary restrictive and financial covenants as well as customary events of default, including financial covenants regarding the maintenance of a minimum level of net worth of $6.6 billion at December 31, 2012 and a maximum leverage ratio of 3.0:1. We are in compliance with the financial covenants, with actual net worth of $8.8 billion and actual leverage ratio of 1.1:1, as measured in accordance with the credit agreement as of December 31, 2012.
Borrowings outstanding under credit agreement		0
Outstanding letters of credit		5,000,000
Remaining borrowing capacity		995,000,000
Junior subordinated debt called	36,000,000	36,000,000	0	0
New Credit Agreement
Debt Instrument [Line Items]
Revolving credit agreement term, in years		5
Amount of unsecured revolving credit agreement		1,000,000,000
Expiration date of credit agreement		2016-11
Facility fee that is dependent upon credit rating, basis points		0.175%
New Credit Agreement | Minimum
Debt Instrument [Line Items]
Facility fee that is dependent upon credit rating, basis points		0.125%
New Credit Agreement | Maximum
Debt Instrument [Line Items]
Facility fee that is dependent upon credit rating, basis points		0.275%
Junior Subordinated Debt From Acquisition | Other Long Term Borrowings
Debt Instrument [Line Items]
Debt instrument, stated interest rate	8.02%
Payment terms	Quarterly
LIBOR
Debt Instrument [Line Items]
Basis points spread over LIBOR	3.10%
LIBOR | New Credit Agreement
Debt Instrument [Line Items]
Basis points spread over LIBOR		1.20%
LIBOR | New Credit Agreement | Minimum
Debt Instrument [Line Items]
Basis points spread over LIBOR		0.875%
LIBOR | New Credit Agreement | Maximum
Debt Instrument [Line Items]
Basis points spread over LIBOR		1.475%
$500 Million, 7.20% Due June 15, 2018
Debt Instrument [Line Items]
Senior notes		500,000,000	500,000,000
Debt instrument, stated interest rate		7.20%	7.20%
Debt instrument, maturity date		Jun. 15, 2018	Jun. 15, 2018
$250 Million, 8.15% Due June 15, 2038
Debt Instrument [Line Items]
Senior notes		250,000,000	250,000,000
Debt instrument, stated interest rate		8.15%	8.15%
Debt instrument, maturity date		Jun. 15, 2038	Jun. 15, 2038
$600 Million, 3.15% Due December 1, 2022
Debt Instrument [Line Items]
Senior notes		600,000,000
Debt instrument, stated interest rate		3.15%
Debt instrument, maturity date		Dec. 01, 2022
$400 Million, 4.625% Due December 1, 2042
Debt Instrument [Line Items]
Senior notes		$ 400,000,000
Debt instrument, stated interest rate		4.625%
Debt instrument, maturity date		Dec. 01, 2042