Long-term Debt - Summary of Long-term Debt (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Debt Instrument [Line Items]
Debt instrument interest rate			6.50%
4.00% Senior Notes due 2025
Debt Instrument [Line Items]
Debt instrument interest rate	4.00%	4.00%
Debt instrument maturity date	2025	2025
3.30% Senior Notes due 2021
Debt Instrument [Line Items]
Debt instrument interest rate	3.30%	3.30%
Debt instrument maturity date	2021	2021
3.00% Senior Notes due 2020
Debt Instrument [Line Items]
Debt instrument interest rate	3.00%	3.00%
Debt instrument maturity date	2020	2020
3.65% Senior Notes due 2023
Debt Instrument [Line Items]
Debt instrument interest rate	3.65%	3.65%
Debt instrument maturity date	2023	2023
4.20% Senior Notes due 2021
Debt Instrument [Line Items]
Debt instrument interest rate	4.20%	4.20%
Debt instrument maturity date	2021	2021
2.40% Senior Notes due 2022
Debt Instrument [Line Items]
Debt instrument interest rate	2.40%	2.40%
Debt instrument maturity date	2022	2022
3.63% Senior Notes due 2022
Debt Instrument [Line Items]
Debt instrument interest rate	3.63%	3.63%
Debt instrument maturity date	2022	2022
2.65% Senior Notes due 2022
Debt Instrument [Line Items]
Debt instrument interest rate	2.65%	2.65%
Debt instrument maturity date	2022	2022
2.20% Senior Notes due 2020
Debt Instrument [Line Items]
Debt instrument interest rate	2.20%	2.20%
Debt instrument maturity date	2020	2020
7.00% Notes due 2038
Debt Instrument [Line Items]
Debt instrument interest rate	7.00%	7.00%
Debt instrument maturity date	2038	2038
4.50% Notes due 2021
Debt Instrument [Line Items]
Debt instrument interest rate	4.50%	4.50%
Debt instrument maturity date	2021	2021
5.95% Notes due 2041
Debt Instrument [Line Items]
Debt instrument interest rate	5.95%	5.95%
Debt instrument maturity date	2041	2041
3.60% Notes due 2022
Debt Instrument [Line Items]
Debt instrument interest rate	3.60%	3.60%
Debt instrument maturity date	2022	2022
5.13% Notes due 2043
Debt Instrument [Line Items]
Debt instrument interest rate	5.13%	5.13%
Debt instrument maturity date	2043	2043
4.00% Notes due 2023
Debt Instrument [Line Items]
Debt instrument interest rate	4.00%	4.00%
Debt instrument maturity date	2023	2023
3.70% Notes due 2024
Debt Instrument [Line Items]
Debt instrument interest rate	3.70%	3.70%
Debt instrument maturity date	2024	2024
0.63% Guaranteed Notes due 2019
Debt Instrument [Line Items]
Debt instrument interest rate	0.63%	0.63%
Debt instrument maturity date	2019	2019
1.50% Guaranteed Notes due 2019
Debt Instrument [Line Items]
Debt instrument interest rate	1.50%	1.50%
Debt instrument maturity date	2019	2019