Debt - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended			0 Months Ended	2 Months Ended	9 Months Ended	3 Months Ended				0 Months Ended	2 Months Ended	9 Months Ended			9 Months Ended	0 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended		3 Months Ended	9 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Senior Notes Extinguished1	Sep. 30, 2013 Senior Notes Extinguished1	Sep. 30, 2013 Senior Notes Seven Point Zero Percent Due February Fifteen Two Thousand Twenty Two	Aug. 26, 2013 Senior Notes Seven Point Zero Percent Due February Fifteen Two Thousand Twenty Two	Mar. 14, 2013 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Aug. 26, 2013 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Sep. 30, 2013 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Sep. 30, 2013 Senior Notes Five Point Five Zero Percent Due May Fifteen Two Thousand Fifteen	Sep. 30, 2012 Senior Notes Five Point Five Zero Percent Due May Fifteen Two Thousand Fifteen	Mar. 31, 2012 Senior Notes Five Point Five Zero Percent Due May Fifteen Two Thousand Fifteen	Mar. 13, 2012 Senior Notes Five Point Five Zero Percent Due May Fifteen Two Thousand Fifteen	Mar. 14, 2013 Senior Notes Six Point One Two Five Percent Due January Fifteen Two Thousand Seventeen	Aug. 26, 2013 Senior Notes Six Point One Two Five Percent Due January Fifteen Two Thousand Seventeen	Sep. 30, 2013 Senior Notes Six Point One Two Five Percent Due January Fifteen Two Thousand Seventeen	Sep. 30, 2013 Senior Notes Seven Point Eight Seven Five Percent Due March Fifteen Two Thousand Twenty One	Mar. 14, 2013 Senior Notes Seven Point Eight Seven Five Percent Due March Fifteen Two Thousand Twenty One	Sep. 30, 2013 Senior Notes Extinguished2	Mar. 14, 2013 Senior Notes Eight Point Six Zero Percent Due August Fifteen Two Thousand Sixteen	Sep. 30, 2013 Senior Notes Eight Point Six Zero Percent Due August Fifteen Two Thousand Sixteen	Mar. 31, 2012 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Sep. 30, 2012 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Apr. 09, 2010 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Mar. 31, 2012 Senior Notes Eight Point Two Five Percent Due March Fifteen Two Thousand Nineteen	Mar. 13, 2012 Senior Notes Eight Point Two Five Percent Due March Fifteen Two Thousand Nineteen	Mar. 31, 2012 Senior Notes Five Point Six Two Five Percent Due January Fifteen Two Thousand Twelve	Sep. 30, 2013 Credit Agreements	Dec. 31, 2012 Credit Agreements	Sep. 30, 2013 Revolving Credit Facility	Sep. 30, 2012 Previous Credit Agreement
Debt Disclosure [Line Items]
Amount of difference between fair value and book value	$ 264,000,000		$ 264,000,000		$ (3,700,000)
Total amount of revolving credit agreement																																	1,150,000,000	1,750,000,000
Weighted average interest rate on borrowings																															2.00%			2.10%
Borrowings outstanding under Credit Agreement																															0	0
Debt instrument, face amount									400,000,000												450,000,000							450,000,000	450,000,000
Interest rate								7.00%				7.25%		5.50%		5.50%			6.125%	7.875%				8.60%	4.95%		4.95%	8.25%	8.25%	5.625%
Maturity date								Feb. 15, 2022				May 15, 2018		May 15, 2015	May 15, 2015				Jan. 15, 2017	Mar. 15, 2021				Aug. 15, 2016	Apr. 01, 2014			Mar. 15, 2019		Jan. 15, 2012
Repayment of Debt			830,200,000	623,600,000						50,000,000	200,000,000		100,000,000		100,000,000		173,500,000	100,000,000					130,200,000		341,800,000					158,600,000
Loss on debt extinguishment	46,300,000		81,900,000	12,100,000		46,300,000	46,300,000															35,600,000
Debt extinguishment - premiums paid, unamortized debt issuance costs and other expenses				23,200,000
Gain due to termination of related interest rate swaps																										11,100,000
Interest income	$ 2,100,000	$ 3,800,000	$ 8,400,000	$ 11,500,000