Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financing
Revolving Credit Facility	$ 0	$ 0
Long-term debt	1,885,877	1,390,018	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	496,153	495,951
Senior notes, unamortized discount	1,700	1,800
Senior notes, unamortized debt issuance costs	$ 2,200	2,300
Senior notes, effective interest rate	4.962%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	$ 298,466	298,396
Senior notes, unamortized discount	300	300
Senior notes, unamortized debt issuance costs	$ 1,200	1,300
Senior notes, effective interest rate	4.647%
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	$ 297,617	297,535
Senior notes, unamortized discount	800	800
Senior notes, unamortized debt issuance costs	$ 1,600	1,700
Senior notes, effective interest rate	3.845%
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	$ 298,190	298,136
Senior notes, unamortized discount	100	100
Senior notes, unamortized debt issuance costs	$ 1,800	$ 1,800
Senior notes, effective interest rate	3.851%
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	$ 495,451
Senior notes, unamortized discount	800
Senior notes, unamortized debt issuance costs	$ 3,700
Senior notes, effective interest rate	3.57%

[1] The balance sheet at December 31, 2015, has been derived from the audited consolidated financial statements at that date, but does not include all of the information and footnotes required by United States generally accepted accounting principles for complete financial statements.