Long-Term Debt And Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended			12 Months Ended
	May 31, 2012	Nov. 30, 2006	Sep. 30, 2003	Dec. 31, 2012
Debt Instrument [Line Items]
Amended and restated credit agreement duration, years	5 years
Amended and restated credit agreement	$ 300
LIBOR plus, variable rate basis				LIBOR rate plus 100 basis points
LIBOR basis points, percentage				1.05%
Federal funds description of variable rate basis				Federal Funds rate plus 50 basis points
Federal funds rate basis spread on variable rate				0.50%
Total debt to capitalization ratio				0.30
Notes issued		200	200
Notes maturity date		Nov. 15, 2036	Sep. 15, 2013
Percentage of principal amount Company may be required to buy back at		101.00%
Date of unsecured notes issued				Apr. 01, 1991
Standby letters of credit				31
Uncommitted letter of credit reimbursement agreement				$ 60
Minimum [Member]
Debt Instrument [Line Items]
LIBOR plus, variable rate basis				LIBOR plus 105 basis points
LIBOR basis points, percentage				1.00%
Maximum [Member]
Debt Instrument [Line Items]
LIBOR plus, variable rate basis				LIBOR plus 147.5 basis points
LIBOR basis points, percentage				1.475%
Total debt to capitalization ratio				0.65
5.50% Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate			5.50%
Annualized interest rate including debt issuance cost amortization				5.70%
6.55% Notes Due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.55%
Annualized interest rate including debt issuance cost amortization				6.67%