Credit Agreement and Long-Term Debt - Summary of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
5.15% Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	$ 250.0
Maturity Date	Sep. 01, 2014
Interest rate of senior notes	5.15%	5.15%
Semiannual Interest Payment Dates	March 1 and September 1
4.875% Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	250.0
Maturity Date	Jul. 01, 2015
Interest rate of senior notes	4.875%	4.875%
Semiannual Interest Payment Dates	January 1 and July 1
5.875% Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	500.0
Maturity Date	May 01, 2019
Interest rate of senior notes	5.875%	5.875%
Semiannual Interest Payment Dates	May 1 and November 1
5.70% Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	$ 500.0
Maturity Date	Oct. 15, 2039
Interest rate of senior notes	5.70%	5.70%
Semiannual Interest Payment Dates	April 15 and October 15