Debt (Debt With A Contractual Term Greater Than One Year) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, net	$ 5,714,527	$ 5,751,859
Current portion of long-term debt, net	(1,127,269)	(1,084,884)
Long-term debt	$ 4,587,258	$ 4,666,975
2.70% Medium-term notes due in 2017
Debt Instrument [Line Items]
Stated interest rate		2.70%
1.55% Medium-term notes due in 2017
Debt Instrument [Line Items]
Stated interest rate		1.55%
6.80% Medium-term notes due 2018
Debt Instrument [Line Items]
Stated interest rate	6.80%
2.40% Medium-term notes due 2019
Debt Instrument [Line Items]
Stated interest rate	2.40%
2.25% Medium-term notes due 2019
Debt Instrument [Line Items]
Stated interest rate	2.25%
2.15% Medium-term notes due 2020
Debt Instrument [Line Items]
Stated interest rate	2.15%
Two Point Four Percent Medium-Term Notes Due 2020 [Member]
Debt Instrument [Line Items]
Stated interest rate	2.40%
Two Point Eighty-five Percent Medium-Term Notes Due 2021 [Member]
Debt Instrument [Line Items]
Stated interest rate	2.85%
Two Point Fifty-five Percent Medium-Term Notes Due 2022 [Member]
Debt Instrument [Line Items]
Stated interest rate	2.55%
3.50% Senior unsecured notes due in 2025
Debt Instrument [Line Items]
Stated interest rate	3.50%
4.625% Senior unsecured notes due in 2045
Debt Instrument [Line Items]
Stated interest rate	4.625%
Secured Debt
Debt Instrument [Line Items]
Less: unamortized discount and debt issuance costs	$ (461)	$ (1,480)
Long-term debt, net	806,860	945,613
Secured Debt | Asset-backed Canadian commercial paper conduit facility
Debt Instrument [Line Items]
Long-term Debt, Gross	174,779	149,338
Secured Debt | Asset-Backed U.S. Commercial Paper Conduit Facility VIE
Debt Instrument [Line Items]
Long-term Debt, Gross	279,457	0
Secured Debt | Term asset-backed securitization debt, VIE
Debt Instrument [Line Items]
Long-term Debt, Gross	353,085	797,755
Senior Unsecured Debt
Debt Instrument [Line Items]
Less: unamortized discount and debt issuance costs	(19,821)	(21,242)
Senior Unsecured Debt | 2.70% Medium-term notes due in 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	0	400,000
Senior Unsecured Debt | 1.55% Medium-term notes due in 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	0	400,000
Senior Unsecured Debt | 6.80% Medium-term notes due 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	877,488	877,488
Senior Unsecured Debt | 2.40% Medium-term notes due 2019
Debt Instrument [Line Items]
Long-term Debt, Gross	600,000	600,000
Senior Unsecured Debt | 2.25% Medium-term notes due 2019
Debt Instrument [Line Items]
Long-term Debt, Gross	600,000	600,000
Senior Unsecured Debt | Floating-Rate Medium Term Notes due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	150,000	0
Senior Unsecured Debt | 2.15% Medium-term notes due 2020
Debt Instrument [Line Items]
Long-term Debt, Gross	600,000	600,000
Senior Unsecured Debt | Two Point Four Percent Medium-Term Notes Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	350,000	0
Senior Unsecured Debt | Two Point Eighty-five Percent Medium-Term Notes Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	600,000	600,000
Senior Unsecured Debt | Two Point Fifty-five Percent Medium-Term Notes Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	400,000	0
Senior Unsecured Debt | 3.50% Senior unsecured notes due in 2025
Debt Instrument [Line Items]
Long-term Debt, Gross	450,000	450,000
Senior Unsecured Debt | 4.625% Senior unsecured notes due in 2045
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 300,000	$ 300,000