Company's Debt Maturities on Calendar Year Basis (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 18, 2013	May 23, 2011
1.50% Convertible Notes
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.50%	1.50%	1.50%
Principal amount of long term debt	$ 400.0		$ 400.0
2.50% Convertible Notes
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	2.50%	2.50%		2.50%
Principal amount of long term debt	$ 300.0			$ 300.0