Fair Value - Fair Value of Debt (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 19, 2016	Dec. 07, 2015	Jun. 06, 2013	Nov. 21, 2012	Sep. 30, 2009
Senior Notes Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 3,900,000,000
$800 million 2.050% senior notes due 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	800,000,000				$ 800,000,000
Fair value of debt	$ 800,000,000	$ 800,000,000
Interest rate	2.05%				2.05%
$800 million 2.050% senior notes due 2018 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 800,000,000
Fair value of debt	$ 800,000,000
Interest rate	2.05%
$500 million 6.250% senior notes due 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 500,000,000						$ 500,000,000
Fair value of debt	$ 500,000,000	500,000,000
Interest rate	6.25%						6.25%
$500 million 6.250% senior notes due 2019 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 500,000,000
Fair value of debt	$ 531,000,000
Interest rate	6.25%
$600 million 3.000% senior notes due 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 600,000,000			$ 600,000,000
Fair value of debt	$ 600,000,000	600,000,000
Interest rate	3.00%			3.00%
$600 million 3.000% senior notes due 2020 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 600,000,000
Fair value of debt	$ 608,000,000
Interest rate	3.00%
$500 million 2.800% senior notes due 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 500,000,000		$ 500,000,000
Fair value of debt	$ 500,000,000	500,000,000
Interest rate	2.80%		2.80%
$500 million 2.800% senior notes due 2021 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 500,000,000
Fair value of debt	$ 501,000,000
Interest rate	2.80%
$500 million 3.125% senior notes due 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 500,000,000					$ 500,000,000
Fair value of debt	$ 500,000,000	500,000,000
Interest rate	3.125%					3.125%
$500 million 3.125% senior notes due 2022 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 500,000,000
Fair value of debt	$ 505,000,000
Interest rate	3.125%
$300 million 3.850% senior notes due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 300,000,000			$ 300,000,000
Fair value of debt	$ 300,000,000	300,000,000
Interest rate	3.85%			3.85%
$300 million 3.850% senior notes due 2025 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 300,000,000
Fair value of debt	$ 311,000,000
Interest rate	3.85%
$700 million 3.800% senior notes due 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 700,000,000		$ 700,000,000
Fair value of debt	$ 700,000,000	$ 700,000,000
Interest rate	3.80%		3.80%
$700 million 3.800% senior notes due 2026 | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of debt	$ 700,000,000
Fair value of debt	$ 723,000,000
Interest rate	3.80%