Long-Term Debt And Notes Payable (Narrative) (Details) (USD $)	1 Months Ended			3 Months Ended	12 Months Ended			1 Months Ended
	Dec. 31, 2012	May 31, 2012	Nov. 30, 2006	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Sep. 30, 2003
Debt Instrument [Line Items]
Repayments of Short-term Debt					$ 25,000,000
Notes maturity date			Nov. 15, 2036
Amended and restated credit agreement duration, years		5 years
Line of credit facility, maximum borrowing capacity		300,000,000				400,000,000		400,000,000
Line of credit facility, amended maximum borrowing capacity						500,000,000		500,000,000
Long-term debt					749,213,000	749,170,000		749,170,000
Line of Credit Facility, Term	364 days
Line of credit facility, borrowing capacity expansion						200,000,000		200,000,000
Total debt to capitalization ratio					0.45
Senior Notes				200,000,000
Repayments of Long-term Debt				90,000,000	0	200,000,000	0
Repayment of Long-term Debt Using Multi-year Credit Facility Borrowings				110,000,000
Date of unsecured notes issued					Apr. 01, 1991
Standby letters of credit					28,000,000
Uncommitted letter of credit reimbursement agreement					125,000,000
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					299,978,000	299,976,000		299,976,000
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, term								5 years
Notes issued								250,000,000
Debt instrument interest rate					2.75%
Long-term debt					249,973,000	249,965,000		249,965,000
Percentage of principal amount Company may be required to buy back at								101.00%
Annualized interest rate including debt issuance cost amortization					2.92%
Bank Loan Commitments [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					600,000,000
Amended and Restated Credit Agreement Due to Expire in May 2017 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding					100,000,000
5.50% Notes Due 2013 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate									5.50%
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					$ 199,262,000	$ 199,229,000		$ 199,229,000
Percentage of principal amount Company may be required to buy back at			101.00%
Annualized interest rate including debt issuance cost amortization					6.67%
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%