Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands			12 Months Ended
		May 17, 2018	Dec. 31, 2018	Dec. 31, 2017
Financing
Senior notes			$ 3,130,122	$ 2,632,390
Senior notes, unamortized discount			4,300	3,700
Senior notes, unamortized debt issuance costs			17,100	15,900
Long-term debt			3,417,122	2,978,390
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount			700	1,100
Senior notes, unamortized debt issuance costs			900	1,400
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount			100	200
Senior notes, unamortized debt issuance costs			600	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			600	700
Senior notes, unamortized debt issuance costs			3,100	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			5,100	5,600
$500 million, 4.350% Senior Notes due 2028 [Member]
Financing
Senior notes, unamortized discount			1,300
Senior notes, unamortized debt issuance costs			3,700
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility			$ 287,000	346,000
Unsecured revolving credit facility, weighted-average variable interest rate			4.56%
Senior notes [Member] | Minimum [Member]
Financing
Interest rate of senior notes			3.55%
Senior notes maturity, year			2021
Senior notes [Member] | Maximum [Member]
Financing
Interest rate of senior notes			4.875%
Senior notes maturity, year			2028
Senior notes [Member] | $500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[1]		$ 498,371	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			4.952%
Senior notes [Member] | $300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	[2]		$ 299,244	$ 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	[3]		$ 298,574	$ 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	[4]		$ 298,821	$ 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	[5]		$ 496,240	$ 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	[6]		$ 743,868	$ 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	[7]		$ 495,004
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] Net of unamortized discount of $0.7 million and $1.1 million as of December 31, 2018 and 2017, respectively, and debt issuance costs of $0.9 million and $1.4 million as of December 31, 2018 and 2017, respectively.

[2] Net of unamortized discount of $0.1 million and $0.2 million as of December 31, 2018 and 2017, respectively, and debt issuance costs of $0.6 million and $0.8 million as of December 31, 2018 and 2017, respectively.

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

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

[5] Net of unamortized discount of $0.6 million and $0.7 million as of December 31, 2018 and 2017, respectively, and debt issuance costs of $3.1 million and $3.5 million as of December 31, 2018 and 2017, respectively.

[6] Net of unamortized discount of $1.1 million and $1.2 million as of December 31, 2018 and 2017, respectively, and debt issuance costs of $5.1 million and $5.6 million as of December 31, 2018 and 2017, respectively.

[7] Net of unamortized discount of $1.3 million as of December 31, 2018, and debt issuance costs of $3.7 million as of December 31, 2018.