Debt, Interest Rate Swap Arrangements (Details) - Interest Rate Swaps [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Notional Amount Of Derivatives	$ 3,100	$ 3,100
Senior Notes [Member] | 4.50% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Notional Amount Of Derivatives	$ 1,000
Interest Rate Swap, Pay Rate Spread above One-month LIBOR	3.442%
Interest Rate Swap, Pay Rate at Period End	5.1062%
Interest Rate Swap, Fixed Receive Rate	4.50%
Senior Notes [Member] | 3.60% Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Notional Amount Of Derivatives	$ 1,100
Interest Rate Swap, Pay Rate Spread above One-month LIBOR	2.515%
Interest Rate Swap, Pay Rate at Period End	4.2916%
Interest Rate Swap, Fixed Receive Rate	3.60%
Senior Notes [Member] | 3.00% Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Notional Amount Of Derivatives	$ 1,000
Interest Rate Swap, Pay Rate Spread above One-month LIBOR	1.764%
Interest Rate Swap, Pay Rate at Period End	3.5406%
Interest Rate Swap, Fixed Receive Rate	3.00%