Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Financing
Senior notes		$ 1,888,062	$ 1,887,019
Long-term debt		2,604,062	1,887,019	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount		1,200	1,400
Senior notes, unamortized debt issuance costs		$ 1,600	1,800
Senior notes, effective interest rate		4.958%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount		$ 200	200
Senior notes, unamortized debt issuance costs		$ 1,000	1,100
Senior notes, effective interest rate		4.646%
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes, unamortized discount		$ 600	700
Senior notes, unamortized debt issuance costs		$ 1,300	1,500
Senior notes, effective interest rate		3.845%
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes, unamortized discount		$ 100	100
Senior notes, unamortized debt issuance costs		$ 1,500	1,600
Senior notes, effective interest rate		3.851%
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes, unamortized discount		$ 700	800
Senior notes, unamortized debt issuance costs		$ 3,700	3,900
Senior notes, effective interest rate		3.57%
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility		$ 716,000	0
Unsecured revolving credit facility, weighted-average variable interest rate		2.065%
Senior notes [Member] | $500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[2]	$ 497,161	496,758
Senior notes, face amount		500,000	500,000
Senior notes [Member] | $300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	[3]	298,820	298,679
Senior notes, face amount		300,000	300,000
Senior notes [Member] | $300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	[4]	298,039	297,868
Senior notes, face amount		300,000	300,000
Senior notes [Member] | $300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	[5]	298,468	298,355
Senior notes, face amount		300,000	300,000
Senior notes [Member] | $500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	[6]	495,574	495,359
Senior notes, face amount		$ 500,000	$ 500,000

[1] The balance sheet at December 31, 2016, 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 $1.2 million as of June 30, 2017, and $1.4 million as of December 31, 2016, and debt issuance costs of $1.6 million as of June 30, 2017, and $1.8 million as of December 31, 2016.

[3] Net of unamortized discount of $0.2 million as of June 30, 2017, and December 31, 2016, and debt issuance costs of $1.0 million as of June 30, 2017, and $1.1 million as of December 31, 2016.

[4] Net of unamortized discount of $0.6 million as of June 30, 2017, and $0.7 million as of December 31, 2016, and debt issuance costs of $1.3 million as of June 30, 2017, and $1.5 million as of December 31, 2016.

[5] Net of unamortized discount of less than $0.1 million as of June 30, 2017, and December 31, 2016, and debt issuance costs of $1.5 million as of June 30, 2017, and $1.6 million as of December 31, 2016.

[6] Net of unamortized discount of $0.7 million as of June 30, 2017, and $0.8 million as of December 31, 2016, and debt issuance costs of $3.7 million as of June 30, 2017, and $3.9 million as of December 31, 2016.