Senior Notes (Schedule Of Corporate Debt) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Jul. 31, 2014	Dec. 31, 2012	Jan. 31, 2012
Debt Instrument [Line Items]
Long-term Debt		$ 750,000		$ 625,000
Stated interest rate		5.375%
Debt instrument, maturity date		Dec. 31, 2022
Senior notes 3.50% due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	[1]	$ 300,000
Stated interest rate		3.50%
Due date		2020
Debt instrument, maturity date		Dec. 31, 2015
Redemption price, percentage of principal amount		100.00%
Senior notes 5.375%, due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 150,000	[2]	150,000	[2]		$ 150,000
Stated interest rate		5.375%					5.375%
Due date		2022
Debt instrument, maturity date		Dec. 31, 2012
Redemption price, percentage of principal amount		100.00%
Senior notes 4.250% due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 300,000	[3]	300,000	[3]	$ 300,000
Stated interest rate		4.25%				4.25%
Due date		2024
Debt instrument, maturity date		Jul. 31, 2014
Redemption price, percentage of principal amount		100.00%
Senior notes 6.70%, due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt				$ 175,000	[4]			$ 175,000
Stated interest rate		6.70%						6.70%
Due date		2022
Debt instrument, maturity date		Jan. 31, 2012
Redemption price, percentage of principal amount		100.00%

[1] In December 2015, we sold in a registered underwritten public offering, $300.0 million in aggregate principal amount of 3.50% senior notes due December 2020. Interest on these senior notes is payable semi-annually in arrears. We may redeem the notes in whole or in part, at our option, at a redemption price equal to 100% of their principal amount, plus a “make-whole” premium and accrued and unpaid interest, if any, to the date of redemption.

[2] In December 2012, we sold in a registered underwritten public offering, $150.0 million in aggregate principal amount of 5.375% senior notes due December 2022. Interest on these senior notes is payable quarterly in arrears. On or after December 31, 2015, we may redeem some or all of the senior notes at any time at a redemption price equal to 100% of the principal amount of the notes being redeemed plus accrued interest thereon to the redemption date.

[3] In July 2014, we sold in a registered underwritten public offering, $300.0 million in aggregate principal amount of 4.250% senior notes due July 2024. Interest on these senior notes is payable semi-annually in arrears. We may redeem the notes in whole or in part, at our option, at a redemption price equal to 100% of their principal amount, plus a “make-whole” premium and accrued and unpaid interest, if any, to the date of redemption.

[4] In January 2012, we sold in a registered underwritten public offering, $175.0 million in aggregate principal amount of 6.70% senior notes due January 2022. Interest on these senior notes is payable quarterly in arrears. On January 15, 2015, we redeemed 100% of our company’s outstanding 6.70% senior notes.