Debt - Summary of Debt (Parenthetical) (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
2.60% Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Interest rate	2.60%	2.60%
Maturity year	2019	2019
4.70% Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Interest rate	4.70%	4.70%
Maturity year	2020	2020
3.15% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate	3.15%	3.15%
Maturity year	2021	2021
3.75% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Interest rate	3.75%	3.75%
Maturity year	2021	2021
4.00% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Maturity year	2022	2022
3.85% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate	3.85%	3.85%
Maturity year	2023	2023
5.00% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Interest rate	5.00%	5.00%
Maturity year	2023	2023
4.00% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Maturity year	2024	2024
3.90% Senior Notes Due 2025 [Member]
Debt Instrument [Line Items]
Interest rate	3.90%	3.90%
Maturity year	2025	2025
4.20% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Interest rate	4.20%	4.20%
Maturity year	2026	2026
5.375% Senior Notes Due 2036 [Member]
Debt Instrument [Line Items]
Interest rate	5.375%	5.375%
Maturity year	2036	2036
5.50% Senior Notes Due 2046 [Member]
Debt Instrument [Line Items]
Interest rate	5.50%	5.50%
Maturity year	2046	2046