Details of Two Point Five Zero Percent Convertible Notes Reflected on Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Equity component carrying amount					$ 95,141
Net debt carrying amount	$ 1,394,077		$ 1,401,783		1,411,122		$ 1,419,084		$ 1,427,319
2.50% Convertible Notes
Debt Instrument [Line Items]
Equity component carrying amount	35,996		35,996		35,996		35,996		35,996
Unamortized discount	19,896		23,300		26,704		29,845		32,987
Net debt carrying amount	$ 280,104	[1]	$ 276,700	[2]	$ 273,296	[3]	$ 270,155	[4]	$ 267,013

[1] Reflects the net debt carrying amount of the 2.50% Convertible Notes in the consolidated balance sheet as of December 31, 2014, in accordance with accounting for convertible notes. The principal amount owed to the holders of the 2.50% Convertible Notes is $300.0 million.

[2] Reflects the net debt carrying amount of the 2.50% Convertible Notes in the consolidated balance sheet as of September 30, 2014, in accordance with accounting for convertible notes. The principal amount owed to the holders of the 2.50% Convertible Notes is $300.0 million.

[3] Reflects the net debt carrying amount of the 2.50% Convertible Notes in the consolidated balance sheet as of June 30, 2014, in accordance with accounting for convertible notes. The principal amount owed to the holders of the 2.50% Convertible Notes is $300.0 million.

[4] Reflects the net debt carrying amount of the 2.50% Convertible Notes in the consolidated balance sheet as of March 31, 2014, in accordance with accounting for convertible notes. The principal amount owed to the holders of the 2.50% Convertible Notes is $300.0 million.