Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Financing
Revolving Credit Facility		$ 0	$ 0
Long-term debt		1,886,501	1,390,018	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[2]	496,556	495,951
Senior notes, unamortized discount		1,500	1,800
Senior notes, unamortized debt issuance costs		$ 1,900	2,300
Senior notes, effective interest rate		4.96%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	[3]	$ 298,608	298,396
Senior notes, unamortized discount		300	300
Senior notes, unamortized debt issuance costs		$ 1,100	1,300
Senior notes, effective interest rate		4.646%
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	[4]	$ 297,784	297,535
Senior notes, unamortized discount		700	800
Senior notes, unamortized debt issuance costs		$ 1,500	1,700
Senior notes, effective interest rate		3.845%
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	[5]	$ 298,299	298,136
Senior notes, unamortized discount		100	100
Senior notes, unamortized debt issuance costs		$ 1,700	$ 1,800
Senior notes, effective interest rate		3.851%
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	[6]	$ 495,254
Senior notes, unamortized discount		800
Senior notes, unamortized debt issuance costs		$ 3,900
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.

[2] Net of unamortized discount of $1.5 million as of September 30, 2016, and $1.8 million as of December 31, 2015, and debt issuance costs of $1.9 million as of September 30, 2016, and $2.3 million as of December 31, 2015.

[3] Net of unamortized discount of $0.3 million as of September 30, 2016, and December 31, 2015, and debt issuance costs of $1.1 million as of September 30, 2016, and $1.3 million as of December 31, 2015.

[4] Net of unamortized discount of $0.7 million as of September 30, 2016, and $0.8 million as of December 31, 2015, and debt issuance costs of $1.5 million as of September 30, 2016, and $1.7 million as of December 31, 2015.

[5] Net of unamortized discount of less than $0.1 million as of September 30, 2016, and December 31, 2015, and debt issuance costs of $1.7 million as of September 30, 2016, and $1.8 million as of December 31, 2015.

[6] Net of unamortized discount of $0.8 million as of September 30, 2016, and debt issuance costs of $3.9 million as of September 30, 2016.