Earnings Per Share - Additional Information (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Mar. 18, 2013	May 23, 2011
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	1.3	0.1		0.1
Convertible Notes | 1.50% Senior Subordinated Notes Due March 15, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Debt instrument, interest rate, stated percentage	1.50%	1.50%	1.50%	1.50%	1.50%	1.50%
Convertible Notes | 2.50% Senior Subordinated Notes Due June 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Debt instrument, interest rate, stated percentage	2.50%	2.50%	2.50%	2.50%	2.50%		2.50%
Performance Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares	0.2
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares			0.1
Maximum | Performance Shares
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive shares		0.1	0.1	0.1