Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Financing
Revolving Credit Facility		$ 0	$ 0
Senior notes, unamortized discount		3,100	2,900
Senior notes, unamortized debt issuance costs		10,600	8,300
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[1]	496,758	495,951
Senior notes, unamortized discount		1,400	1,800
Senior notes, unamortized debt issuance costs		$ 1,800	2,300
Senior notes, effective interest rate		4.959%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	[2]	$ 298,679	298,396
Senior notes, unamortized discount		200	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	[3]	$ 297,868	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	[4]	$ 298,355	298,136
Senior notes, unamortized discount		100	100
Senior notes, unamortized debt issuance costs		$ 1,600	$ 1,800
Senior notes, effective interest rate		3.851%
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	[5]	$ 495,359
Senior notes, unamortized discount		800
Senior notes, unamortized debt issuance costs		$ 3,900
Senior notes, effective interest rate		3.57%

[1] Net of unamortized discount of $1.4 million and $1.8 million as of December 31, 2016 and 2015, respectively, and debt issuance costs of $1.8 million and $2.3 million as of December 31, 2016 and 2015, respectively.

[2] Net of unamortized discount of $0.2 million and $0.3 million as of December 31, 2016 and 2015, respectively, and debt issuance costs of $1.1 million and $1.3 million as of December 31, 2016 and 2015, respectively.

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

[4] Net of unamortized discount of less than $0.1 million as of December 31, 2016 and 2015, and debt issuance costs of $1.6 million and $1.8 million as of December 31, 2016 and 2015, respectively.

[5] Net of unamortized discount of $0.8 million as of December 31, 2016, and debt issuance costs of $3.9 million as of December 31, 2016.