Long-Term Debt And Notes Payable (Narrative) (Details) - USD ($)		1 Months Ended			9 Months Ended	12 Months Ended
		Dec. 31, 2013	May. 31, 2012	Nov. 30, 2006	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2003
Debt Instrument [Line Items]
Amended and restated credit agreement duration, years			5 years
Line of credit facility, maximum borrowing capacity			$ 300,000,000
Line of credit facility, amended maximum borrowing capacity					$ 500,000,000		$ 500,000,000
Long-term debt						$ 749,300,000	749,200,000
Total debt to capitalization ratio						0.41
Repayments of Long-term Debt						$ 0	$ 0	$ 200,000,000
Standby letters of credit						23,000,000
Uncommitted letter of credit reimbursement agreement						$ 125,000,000
Long-term Commercial Paper					$ 500,000,000
number of days to maturity					397
Facility fee					0.15%
Margin					0.10%
Rate for LIBOR loans					1.10%
Maximum [Member]
Debt Instrument [Line Items]
Total debt to capitalization ratio						0.65
4.45% Notes Due 2023 [Member]
Debt Instrument [Line Items]
Notes issued, term		10 years
Notes issued		$ 300,000,000
Debt instrument interest rate							4.45%
Long-term debt						$ 199,300,000	$ 1,199,200,000
Percentage of principal amount Company may be required to buy back at		101.00%
Annualized interest rate including debt issuance cost amortization						4.56%
2.75% Notes Due 2018 [Member]
Debt Instrument [Line Items]
Notes issued		$ 250,000,000
Debt instrument interest rate							2.75%
Long-term debt						$ 300,000,000	$ 300,000,000
Percentage of principal amount Company may be required to buy back at		101.00%
Annualized interest rate including debt issuance cost amortization						2.92%
5.50% Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate									5.50%
Long-term debt	[1]					$ 250,000,000	250,000,000
6.55% Notes Due 2036 [Member]
Debt Instrument [Line Items]
Notes issued, term				30 years
Notes issued				$ 200,000,000
Debt instrument interest rate				6.55%
Long-term debt						$ 749,300,000	$ 749,200,000
Percentage of principal amount Company may be required to buy back at				101.00%
Annualized interest rate including debt issuance cost amortization						6.67%
Commercial Paper [Member]
Debt Instrument [Line Items]
Long-term debt						$ 49,000,000
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						1.05%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						1.475%
Federal Funds Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						0.50%
Adjusted London Interbank Offered Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						1.00%
Adjusted London Interbank Offered Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						0.05%
Adjusted London Interbank Offered Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						0.475%

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