Debt - Narrative (Detail) (USD $)	3 Months Ended		9 Months Ended			0 Months Ended	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended		0 Months Ended		9 Months Ended					9 Months Ended		9 Months Ended	0 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended		0 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Apr. 09, 2010 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Mar. 31, 2014 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Mar. 31, 2012 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Sep. 30, 2014 Senior Notes Four Point Nine Five Percent Due April One Two Thousand Fourteen	Sep. 30, 2014 Senior Notes Six Point Zero Zero Percent Due April One Two Thousand Twenty Four	Mar. 20, 2014 Senior Notes Six Point Zero Zero Percent Due April One Two Thousand Twenty Four	Mar. 31, 2012 Senior Notes Eight Point Two Five Percent Due March Fifteen Two Thousand Nineteen	Sep. 30, 2014 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	Aug. 26, 2013 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Mar. 14, 2013 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Sep. 30, 2014 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Sep. 30, 2014 Senior Notes Seven Point Six Two Five Percent Due June Fifteen Two Thousand Twenty	Sep. 30, 2014 Debt Instruments Extinguished Three	Sep. 30, 2014 Senior Notes Six Point Five Zero Percent Due November Fifteen Two Thousand Twenty Three	Nov. 12, 2013 Senior Notes Six Point Five Zero Percent Due November Fifteen Two Thousand Twenty Three	Sep. 30, 2014 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	Sep. 30, 2014 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	Sep. 30, 2014 Senior Notes Six Point One Two Five Percent Due January Fifteen Two Thousand Seventeen	Dec. 31, 2013 Debt Instruments Extinguished One	Sep. 30, 2014 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	Mar. 14, 2013 Senior Notes Eight Point Six Zero Percent Due August Fifteen Two Thousand Sixteen	Sep. 30, 2014 Senior Notes Eight Point Six Zero Percent Due August Fifteen Two Thousand Sixteen	Mar. 31, 2013 Debt Instruments Extinguished Two	Sep. 30, 2014 Revolving Credit Facility	Sep. 30, 2013 Revolving Credit Facility	Sep. 09, 2014 Revolving Credit Facility
Debt Instrument [Line Items]
Amount of difference between fair value and book value	$ 306,900,000		$ 306,900,000		$ 343,400,000
Total amount of revolving credit agreement																																			1,500,000,000
Line of credit facility previous borrowing capacity																																			1,150,000,000
Line of credit facility, expiration date																																	Sep. 09, 2019
Line of credit facility expiration date prior to amendment																																	Oct. 15, 2017
Line of credit facility amendment date																																	Sep. 09, 2014
Weighted average interest rate on borrowings																																	2.00%	2.00%
Repayment of Debt			811,300,000	830,200,000			258,200,000	341,800,000					211,100,000		200,000,000	50,000,000	100,000,000	50,000,000							173,500,000	100,000,000				130,200,000
Interest rate						4.95%		4.95%	4.95%	6.00%			8.25%	8.25%			7.25%	7.625%		6.50%		7.00%		5.50%		6.125%		7.875%			8.60%
Maturity date						Apr. 01, 2014		Apr. 01, 2014	Apr. 01, 2014	Apr. 01, 2024		Mar. 15, 2019	Mar. 15, 2019				May 15, 2018	Jun. 15, 2020		Nov. 15, 2023		Feb. 15, 2022		May 15, 2015		Jan. 15, 2017		Mar. 15, 2021			Aug. 15, 2016
Debt instrument, face amount											400,000,000		238,900,000	450,000,000							350,000,000		400,000,000						450,000,000
Loss on debt extinguishment	0	(46,300,000)	(77,100,000)	(81,900,000)															77,100,000								46,300,000					35,600,000
Gain due to termination of related interest rate swaps																			2,800,000
Interest income	$ 1,900,000	$ 2,100,000	$ 6,600,000	$ 8,400,000