Long-Term Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended					12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Covenant Requirement Minimum	Dec. 31, 2012 Covenant Requirement Maximum	Dec. 31, 2012 Senior Unsecured Notes 6.875 Percent Due 2020	Dec. 31, 2012 Senior Unsecured Notes 5.25 Percent	Dec. 31, 2012 Senior Unsecured Notes 5.25 Percent Due 2022	Mar. 13, 2012 Senior Unsecured Notes 5.25 Percent Due 2022	Dec. 31, 2012 Senior Unsecured Notes 4.9 percent	Jul. 12, 2012 Senior Unsecured Notes 4.9 percent	Dec. 31, 2012 Senior Unsecured Notes 8.5 Percent Due 2018	Dec. 31, 2012 Senior Unsecured Notes 5.0 percent
Debt Instrument [Line Items]
Senior notes, aggregate principal amount						$ 650.0	$ 1,300.0		$ 500.0		$ 800.0
Debt, interest rate						6.875%	5.25%	5.25%		4.90%		8.50%	5.00%
Debt, due date						2020		2022				2018
Principal payments on long-term debt	600.5	0.5	418.9									600.0
Loss on extinguishment of debt	(82.1)		(12.4)									82.1
Senior notes, net of original issue discount or premiums						645.2	1,315.0
Revolving credit facility	950.0
Revolving credit facility, interest rate, additional to LIBOR	2.00%
Revolving credit facility, interest rate, additional to Prime	1.00%
Revolving credit facility agreement	Borrowings under the Revolving Credit Facility bear interest at an annual rate equal to the London interbank offered rate ("LIBOR") (as defined in the Revolving Credit Facility) plus 200 basis points or Prime (as defined in the Revolving Credit Facility) plus 100 basis points.
Revolving credit facility, average interest rate	2.30%
Outstanding letter of credit amount	11.6
Interest coverage ratio				2.0
Total leverage ratio					4.25
Long-term debt discount amount						4.8
Long -term debt premium amount							15.0
Interest expense	$ 124.6	$ 105.5	$ 92.7