Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands			12 Months Ended
		Aug. 17, 2017	Dec. 31, 2017	Dec. 31, 2016
Financing
Senior notes			$ 2,632,390	$ 1,887,019
Senior notes, unamortized discount			3,700	3,100
Senior notes, unamortized debt issuance costs			15,900	10,600
Long-term debt			2,978,390	1,887,019
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount			1,100	1,400
Senior notes, unamortized debt issuance costs			$ 1,400	$ 1,800
Interest rate of senior notes			4.875%	4.875%
Senior notes maturity, year			2021	2021
Senior notes, effective interest rate			4.956%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount			$ 200	$ 200
Senior notes, unamortized debt issuance costs			$ 800	$ 1,100
Interest rate of senior notes			4.625%	4.625%
Senior notes maturity, year			2021	2021
Senior notes, effective interest rate			4.645%
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes, unamortized discount			$ 600	$ 700
Senior notes, unamortized debt issuance costs			$ 1,200	$ 1,500
Interest rate of senior notes			3.80%	3.80%
Senior notes maturity, year			2022	2022
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			$ 1,400	$ 1,600
Interest rate of senior notes			3.85%	3.85%
Senior notes maturity, year			2023	2023
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			$ 3,500	$ 3,900
Interest rate of senior notes			3.55%	3.55%
Senior notes maturity, year			2026	2026
Senior notes, effective interest rate			3.57%
$750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes, unamortized discount			$ 1,200
Senior notes, unamortized debt issuance costs			$ 5,600
Interest rate of senior notes			3.60%
Senior notes maturity, year			2027
Senior notes, effective interest rate			3.619%
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility			$ 346,000	$ 0
Unsecured revolving credit facility, weighted-average variable interest rate			2.675%
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			2027
Senior notes [Member] | $500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[1]		$ 497,565	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	[2]		298,961	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	[3]		298,214	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	[4]		298,583	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	[5]		495,792	495,359
Senior notes, face amount			500,000	$ 500,000
Senior notes [Member] | $750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes	[6]		743,275
Senior notes, face amount		$ 750,000	$ 750,000
Interest rate of senior notes		3.60%
Senior notes maturity, year		2027

[1] Net of unamortized discount of $1.1 million and $1.4 million as of December 31, 2017 and 2016, respectively, and debt issuance costs of $1.4 million and $1.8 million as of December 31, 2017 and 2016, respectively.

[2] Net of unamortized discount of $0.2 million and $0.2 million as of December 31, 2017 and 2016, respectively, and debt issuance costs of $0.8 million and $1.1 million as of December 31, 2017 and 2016, respectively.

[3] Net of unamortized discount of $0.6 million and $0.7 million as of December 31, 2017 and 2016, respectively, and debt issuance costs of $1.2 million and $1.5 million as of December 31, 2017 and 2016, respectively.

[4] Net of unamortized discount of less than $0.1 million as of December 31, 2017 and 2016, and debt issuance costs of $1.4 million and $1.6 million as of December 31, 2017 and 2016, respectively.

[5] Net of unamortized discount of $0.7 million and $0.8 million as of December 31, 2017 and 2016, respectively, and debt issuance costs of $3.5 million and $3.9 million as of December 31, 2017 and 2016, respectively.

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