Debt (Debt With A Contractual Term Greater Than One Year) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Unsecured commercial paper	$ 0	$ 35,800
Credit facilities	0	159,794
Gross long-term debt	4,807,706	4,884,133
Long-term Debt, Current Maturities	(437,162)	(1,040,247)
Long-term debt	4,370,544	3,843,886
Par value of debt instrument	675,300	1,090,000
Secured Debt		2,087,346
2.70% Medium-Term Notes Due In 2017 ($400.0 Million Par Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	399,929	0
1.15%_Medium-term_notes_due_in_2015_($600.0 Million_Par_Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	599,269	0
5.25% Medium-Term Notes Due In 2012 ($400.0 Million Par Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	0	399,916
5.75% Medium-Term Notes Due In 2014 ($500.0 Million Par Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	499,705	499,544
3.88% Medium-term notes Due In 2016 ($450.0 Million Par Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	449,829	449,775
6.80% Medium-Term Notes Due In 2018 ($950.1 Million Par Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	932,540	948,958
15.00% Senior Unsecured Notes Due In 2014 ($600.0 Million Par Value) [Member]
Debt Instrument [Line Items]
Unsecured notes	303,000	303,000
Term Asset-Backed Securitization Debt [Member]
Debt Instrument [Line Items]
Secured Debt	1,447,776	2,087,346
Asset-backed Canadian Commercial Paper Conduit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	(37,700)
Maturity Date Of Debt Instrument	5 years
Secured Debt	$ 175,658	$ 0