Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands			9 Months Ended	12 Months Ended
		May 17, 2018	Sep. 30, 2018	Dec. 31, 2017
Financing
Senior notes			$ 3,129,327	$ 2,632,390
Long-term debt			3,174,327	2,978,390	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount			800	1,100
Senior notes, unamortized debt issuance costs			1,000	1,400
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount			200	200
Senior notes, unamortized debt issuance costs			700	800
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes, unamortized discount			500	600
Senior notes, unamortized debt issuance costs			1,000	1,200
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes, unamortized discount			100	100
Senior notes, unamortized debt issuance costs			1,200	1,400
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes, unamortized discount			700	700
Senior notes, unamortized debt issuance costs			3,200	3,500
$750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes, unamortized discount			1,100	1,200
Senior notes, unamortized debt issuance costs			5,200	5,600
$500 million, 4.350% Senior Notes due 2028 [Member]
Financing
Senior notes, unamortized discount			1,300
Senior notes, unamortized debt issuance costs			3,800
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility			$ 45,000	346,000
Unsecured revolving credit facility, weighted-average variable interest rate			4.044%
Senior notes [Member] | $500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[2]		$ 498,170	497,565
Senior notes, face amount			$ 500,000	$ 500,000
Interest rate of senior notes			4.875%	4.875%
Senior notes maturity, year			2021	2021
Senior notes, effective interest rate			4.953%
Senior notes [Member] | $300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	[3]		$ 299,173	$ 298,961
Senior notes, face amount			$ 300,000	$ 300,000
Interest rate of senior notes			4.625%	4.625%
Senior notes maturity, year			2021	2021
Senior notes, effective interest rate			4.645%
Senior notes [Member] | $300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	[4]		$ 298,482	$ 298,214
Senior notes, face amount			$ 300,000	$ 300,000
Interest rate of senior notes			3.80%	3.80%
Senior notes maturity, year			2022	2022
Senior notes, effective interest rate			3.845%
Senior notes [Member] | $300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	[5]		$ 298,760	$ 298,583
Senior notes, face amount			$ 300,000	$ 300,000
Interest rate of senior notes			3.85%	3.85%
Senior notes maturity, year			2023	2023
Senior notes, effective interest rate			3.851%
Senior notes [Member] | $500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	[6]		$ 496,126	$ 495,792
Senior notes, face amount			$ 500,000	$ 500,000
Interest rate of senior notes			3.55%	3.55%
Senior notes maturity, year			2026	2026
Senior notes, effective interest rate			3.57%
Senior notes [Member] | $750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes	[7]		$ 743,718	$ 743,275
Senior notes, face amount			$ 750,000	$ 750,000
Interest rate of senior notes			3.60%	3.60%
Senior notes maturity, year			2027	2027
Senior notes, effective interest rate			3.619%
Senior notes [Member] | $500 million, 4.350% Senior Notes due 2028 [Member]
Financing
Senior notes	[8]		$ 494,898
Senior notes, face amount		$ 500,000	$ 500,000
Interest rate of senior notes		4.35%	4.35%
Senior notes maturity, year		2028	2028
Senior notes, effective interest rate			4.383%

[1] The balance sheet at December 31, 2017, 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 $0.8 million as of September 30, 2018, and $1.1 million as of December 31, 2017, and debt issuance costs of $1.0 million as of September 30, 2018, and $1.4 million as of December 31, 2017.

[3] Net of unamortized discount of $0.2 million as of September 30, 2018, and December 31, 2017, and debt issuance costs of $0.7 million as of September 30, 2018, and $0.8 million as of December 31, 2017.

[4] Net of unamortized discount of $0.5 million as of September 30, 2018, and $0.6 million as of December 31, 2017, and debt issuance costs of $1.0 million as of September 30, 2018, and $1.2 million as of December 31, 2017.

[5] Net of unamortized discount of less than $0.1 million as of September 30, 2018, and December 31, 2017, and debt issuance costs of $1.2 million as of September 30, 2018, and $1.4 million as of December 31, 2017.

[6] Net of unamortized discount of $0.7 million as of September 30, 2018, and December 31, 2017, and debt issuance costs of $3.2 million as of September 30, 2018, and $3.5 million as of December 31, 2017.

[7] Net of unamortized discount of $1.1 million as of September 30, 2018, and $1.2 million as of December 31, 2017, and debt issuance costs of $5.2 million as of September 30, 2018 and $5.6 million as of December 31, 2017.

[8] Net of unamortized discount of $1.3 million as of September 30, 2018, and debt issuance costs of $3.8 million as of September 30, 2018.