Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing
Senior notes	$ 3,130,921	$ 3,130,122
Long-term debt	3,460,921	3,417,122	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	498,573	498,371
Senior notes, face amount	$ 500,000	$ 500,000
Interest rate of senior notes	4.875%	4.875%
Senior notes, effective interest rate	4.952%
Senior notes, unamortized discount	$ 600	$ 700
Senior notes, unamortized debt issuance costs	800	900
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	299,315	299,244
Senior notes, face amount	$ 300,000	$ 300,000
Interest rate of senior notes	4.625%	4.625%
Senior notes, effective interest rate	4.644%
Senior notes, unamortized discount	$ 100	$ 100
Senior notes, unamortized debt issuance costs	600	600
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	298,666	298,574
Senior notes, face amount	$ 300,000	$ 300,000
Interest rate of senior notes	3.80%	3.80%
Senior notes, effective interest rate	3.845%
Senior notes, unamortized discount	$ 400	$ 500
Senior notes, unamortized debt issuance costs	900	1,000
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	298,881	298,821
Senior notes, face amount	$ 300,000	$ 300,000
Interest rate of senior notes	3.85%	3.85%
Senior notes, effective interest rate	3.851%
Senior notes, unamortized discount	$ 100	$ 100
Senior notes, unamortized debt issuance costs	1,100	1,200
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	496,354	496,240
Senior notes, face amount	$ 500,000	$ 500,000
Interest rate of senior notes	3.55%	3.55%
Senior notes, effective interest rate	3.57%
Senior notes, unamortized discount	$ 600	$ 600
Senior notes, unamortized debt issuance costs	3,000	3,100
$750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes	744,020	743,868
Senior notes, face amount	$ 750,000	$ 750,000
Interest rate of senior notes	3.60%	3.60%
Senior notes, effective interest rate	3.619%
Senior notes, unamortized discount	$ 1,000	$ 1,100
Senior notes, unamortized debt issuance costs	4,900	5,100
$500 million, 4.350% Senior Notes due 2028 [Member]
Financing
Senior notes	495,112	495,004
Senior notes, face amount	$ 500,000	$ 500,000
Interest rate of senior notes	4.35%	4.35%
Senior notes, effective interest rate	4.383%
Senior notes, unamortized discount	$ 1,200	$ 1,300
Senior notes, unamortized debt issuance costs	3,600	3,700
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility	$ 330,000	$ 287,000
Unsecured revolving credit facility, weighted-average variable interest rate	3.74%

[1] The balance sheet at December 31, 2018, 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.