Senior Notes (Schedule Of Corporate Debt) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015	Jul. 31, 2014	Dec. 31, 2012
Debt Instrument [Line Items]
Long-term Debt, gross		$ 750,000	$ 750,000
Debt issuance costs		(9,591)	(9,864)
Long-term Debt		$ 740,409	740,136
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, gross	[1]	$ 300,000	300,000
Long-term Debt			$ 300,000
Stated interest rate		3.50%	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, gross	[2]	$ 150,000	$ 150,000
Long-term Debt					$ 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, gross	[3]	$ 300,000	$ 300,000
Long-term Debt				$ 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%

[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. 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.