Credit Agreement and Senior Notes - Summary of Senior Notes (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Semiannual Interest Payment Dates	May 1 and November 1
5.15% Senior Notes due 2014 [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	$ 250.0
Maturity Date	Sep. 01, 2014
Interest rate Coupon, senior notes	5.15%	5.15%
Interest rate Effective, senior notes	5.18%
Semiannual Interest Payment Dates	March 1 and September 1
4.875% Senior Notes due 2015 [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	250.0
Maturity Date	Jul. 01, 2015
Interest rate Coupon, senior notes	4.875%	4.875%
Interest rate Effective, senior notes	4.90%
Semiannual Interest Payment Dates	January 1 and July 1
5.875% Senior Notes due 2019 [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	500.0
Maturity Date	May 01, 2019
Interest rate Coupon, senior notes	5.875%	5.875%
Interest rate Effective, senior notes	5.89%
Semiannual Interest Payment Dates	May 1 and November 1
3.45% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	250.0
Maturity Date	Nov. 01, 2023
Interest rate Coupon, senior notes	3.45%
Interest rate Effective, senior notes	3.50%
Semiannual Interest Payment Dates	May 1 and November 1
5.70% Senior Notes due 2039 [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	500.0
Maturity Date	Oct. 15, 2039
Interest rate Coupon, senior notes	5.70%	5.70%
Interest rate Effective, senior notes	5.75%
Semiannual Interest Payment Dates	April 15 and October 15
4.875% Senior Notes due 2043 [Member]
Debt Instrument [Line Items]
Aggregate Principal Amount	$ 750.0
Maturity Date	Nov. 01, 2043
Interest rate Coupon, senior notes	4.875%
Interest rate Effective, senior notes	4.89%
Semiannual Interest Payment Dates	May 1 and November 1