Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Parenthetical) (Details)	Sep. 30, 2018	Dec. 31, 2017
2.45% Senior notes due December 15, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	2.45%	2.45%
2.45% Senior notes due December 15, 2019 | Fair Value, Inputs, Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	2.45%	2.45%
2.45% Senior notes due August 1, 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	2.45%	2.45%
2.45% Senior notes due August 1, 2022 | Fair Value, Inputs, Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	2.45%	2.45%
2.875% Senior notes due October 1, 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	2.875%	2.875%
2.875% Senior notes due October 1, 2022 | Fair Value, Inputs, Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	2.875%	2.875%
3.15% Senior notes due August 1, 2027
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	3.15%	3.15%
3.15% Senior notes due August 1, 2027 | Fair Value, Inputs, Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	3.15%	3.15%
3.95% Senior notes due August 1, 2047
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	3.95%	3.95%
3.95% Senior notes due August 1, 2047 | Fair Value, Inputs, Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Interest rate of debt	3.95%	3.95%