Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financing
Revolving Credit Facility	$ 0	$ 0
Senior notes, unamortized discount	2,900	3,400
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	495,951	495,144	[1]
Senior notes, unamortized discount	1,800	2,100
Senior notes, unamortized debt issuance costs	$ 2,300	2,700
Senior notes, effective interest rate	4.963%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	$ 298,396	298,113	[1]
Senior notes, unamortized discount	300	400
Senior notes, unamortized debt issuance costs	$ 1,300	1,500
Senior notes, effective interest rate	4.647%
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	$ 297,535	297,215	[1]
Senior notes, unamortized discount	800	900
Senior notes, unamortized debt issuance costs	$ 1,700	1,900
Senior notes, effective interest rate	3.845%
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	$ 298,136	297,925	[1]
Senior notes, unamortized discount	100	100
Senior notes, unamortized debt issuance costs	$ 1,800	$ 2,100
Senior notes, effective interest rate	3.851%

[1] Prior period amounts have been reclassified to conform to current period presentation, due to the Company's adoption of new accounting standards during the fourth quarter ended December 31, 2015, see Note 1 for further information.