Debt - Additional Information (Detail) (USD $)	6 Months Ended	1 Months Ended
	Jun. 30, 2013	Jul. 31, 2013 Subsequent Event Prior Credit Agreement Y	Jul. 31, 2013 Subsequent Event New Credit Agreement Y	Jul. 31, 2013 Subsequent Event New Credit Agreement Minimum	Jul. 31, 2013 Subsequent Event New Credit Agreement Maximum	Jul. 31, 2013 Subsequent Event LIBOR New Credit Agreement	Jul. 31, 2013 Subsequent Event LIBOR New Credit Agreement Minimum	Jul. 31, 2013 Subsequent Event LIBOR New Credit Agreement Maximum
Debt Instrument [Line Items]
Revolving credit agreement term, in years		5	5
Amount of unsecured revolving credit agreement		$ 1,000,000,000	$ 1,000,000,000
Expiration date of credit agreement		2016-11	2018-07
Basis points spread over LIBOR						1.10%	0.90%	1.50%
Facility fee that is dependent upon credit rating, basis points			0.15%	0.10%	0.25%
Minimum level of net worth required	7,100,000,000
Maximum leverage ratios for new credit agreement	3.0
Actual net worth	9,300,000,000
Actual leverage ratio	0.9
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 $7.1 billion at June 30, 2013 and a maximum leverage ratio of 3.0:1. We are in compliance with the financial covenants, with actual net worth of $9.3 billion and an actual leverage ratio of 0.9:1, as measured in accordance with the credit agreement as of June 30, 2013.
Borrowings outstanding under credit agreement	0
Outstanding letters of credit	5,500,000
Remaining borrowing capacity	$ 994,500,000