Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Financing
Senior notes		$ 2,633,066	$ 2,632,390
Long-term debt		3,193,066	2,978,390	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount		1,000	1,100
Senior notes, unamortized debt issuance costs		$ 1,300	$ 1,400
Interest rate of senior notes		4.875%	4.875%
Senior notes maturity, year		2021	2021
Senior notes, effective interest rate		4.955%
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes, unamortized discount		$ 200	$ 200
Senior notes, unamortized debt issuance costs		$ 800	$ 800
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		$ 500	$ 600
Senior notes, unamortized debt issuance costs		$ 1,200	$ 1,200
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		$ 1,300	$ 1,400
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	$ 700
Senior notes, unamortized debt issuance costs		$ 3,400	$ 3,500
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		$ 1,100	$ 1,200
Senior notes, unamortized debt issuance costs		$ 5,400	$ 5,600
Interest rate of senior notes		3.60%	3.60%
Senior notes maturity, year		2027	2027
Senior notes, effective interest rate		3.619%
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility		$ 560,000	$ 346,000
Unsecured revolving credit facility, weighted-average variable interest rate		2.784%
Senior notes [Member] | $500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	[2]	$ 497,766	497,565
Senior notes, face amount		500,000	500,000
Senior notes [Member] | $300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	[3]	299,032	298,961
Senior notes, face amount		300,000	300,000
Senior notes [Member] | $300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	[4]	298,303	298,214
Senior notes, face amount		300,000	300,000
Senior notes [Member] | $300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	[5]	298,642	298,583
Senior notes, face amount		300,000	300,000
Senior notes [Member] | $500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	[6]	495,902	495,792
Senior notes, face amount		500,000	500,000
Senior notes [Member] | $750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes	[7]	743,421	743,275
Senior notes, face amount		$ 750,000	$ 750,000

[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 $1.0 million as of March 31, 2018, and $1.1 million as of December 31, 2017, and debt issuance costs of $1.3 million as of March 31, 2018, and $1.4 million as of December 31, 2017.

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

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

[5] Net of unamortized discount of less than $0.1 million as of March 31, 2018, and December 31, 2017, and debt issuance costs of $1.3 million as of March 31, 2018, and $1.4 million as of December 31, 2017.

[6] Net of unamortized discount of $0.7 million as of March 31, 2018, and December 31, 2017, and debt issuance costs of $3.4 million as of March 31, 2018, and $3.5 million as of December 31, 2017.

[7] Net of unamortized discount of $1.1 million as of March 31, 2018, and $1.2 million as of December 31, 2017, and debt issuance costs of $5.4 million as of March 31, 2018 and $5.6 million as of December 31, 2017.