Debt - Additional Information (Detail) (USD $)	3 Months Ended			6 Months Ended			6 Months Ended		6 Months Ended						3 Months Ended			3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Credit Agreement	Dec. 31, 2012 Credit Agreement	Jun. 30, 2012 Previous Credit Agreement	Jun. 30, 2013 Senior Notes Seven Point Eight Seven Five Percent Due March Fifteen Two Thousand Twenty One	Jun. 30, 2013 Senior Notes Six Point One Two Five Percent Due January Fifteen Two Thousand Seventeen	Jun. 30, 2013 Senior Notes Eight Point Six Zero Percent Due August Fifteen Two Thousand Sixteen	Jun. 30, 2013 Senior Notes Seven Point Two Five Percent Due May Fifteen Two Thousand Eighteen	Jun. 30, 2013 Revolving Credit Facility	Mar. 31, 2012 Senior Notes Eight Point Twenty Five Percent Due March Fifteen Two Thousand Nineteen	Mar. 31, 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 Five Point Five Zero Percent Due May Fifteen Two Thousand Fifteen	Mar. 31, 2012 Senior Notes Five Point Six Two Five Percent Due January Fifteen Two Thousand Twelve
Debt Disclosure [Line Items]
Amount of difference between fair value and book value	$ 163,100,000			$ 163,100,000		$ (3,700,000)
Debt instrument, face amount										450,000,000					450,000,000
Interest rate										7.875%	6.125%	8.60%	7.25%		8.25%	4.95%	4.95%	5.50%	5.625%
Maturity date										Mar. 15, 2021	Jan. 15, 2017	Aug. 15, 2016	May 15, 2018		Mar. 15, 2019	Apr. 01, 2014		May 15, 2015	Jan. 15, 2012
Repayment of Debt											173,500,000	130,200,000	50,000,000			341,800,000		100,000,000	158,600,000
Total amount of revolving credit agreement						1,750,000,000								1,150,000,000
Pre-tax loss on debt extinguishment				35,600,000	12,100,000
Pre-tax loss on debt extinguishment			23,200,000
Gain due to termination of the related interest rate swaps			11,100,000
Interest income	2,500,000	4,100,000		6,300,000	7,700,000
Weighted average interest rate on borrowings							2.04%		2.07%
Borrowings outstanding under Credit Agreement							$ 0	$ 0