Financing Arrangements - Net Carrying Amount of the Liability Component and the Remaining Unamortized Debt Discount (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Carrying amount of the liability component	$ 1,021,488	$ 1,024,100	$ 1,026,713	$ 1,110,458	$ 1,114,481	$ 1,146,585	$ 706,420	$ 109,825	$ 108,750
Convertible Debt [Member]
Debt Instrument [Line Items]
Carrying amount of equity component	7,372				14,930
Carrying amount of the liability component	42,465				82,543
Unamortized discount of the liability component	750				4,982
Unamortized debt financing costs	$ 136				$ 901