Fair Value Measurements - Details of Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
5.375% Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate on Notes	5.375%	5.375%
1.125% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate on Notes	1.125%	1.125%
1.625% Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of contractual interest rate on Notes	1.625%	1.625%
Level 2 | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	$ 1,437	$ 1,410
Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	1,910	1,930
Level 2 | 5.375% Notes | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	690	689
Level 2 | 5.375% Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	725	700
Level 2 | 1.125% Convertible Notes | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	466	448
Level 2 | 1.125% Convertible Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	827	865
Level 2 | 1.625% Convertible Notes | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	281	273
Level 2 | 1.625% Convertible Notes | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of debt	$ 358	$ 365