Financing (Outstanding Financing Facilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	May 20, 2019	Dec. 31, 2018
Financing
Senior notes	$ 3,627,738		$ 3,130,122
Long-term debt	3,783,738		3,417,122	[1]
$500 million, 4.875% Senior Notes due 2021 [Member]
Financing
Senior notes	498,775		498,371
Senior notes, face amount	$ 500,000		$ 500,000
Interest rate of senior notes	4.875%		4.875%
Senior notes, effective interest rate	4.951%
Senior notes, unamortized discount	$ 500		$ 700
Senior notes, unamortized debt issuance costs	700		900
$300 million, 4.625% Senior Notes due 2021 [Member]
Financing
Senior notes	299,385		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	500		600
$300 million, 3.800% Senior Notes due 2022 [Member]
Financing
Senior notes	298,759		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	800		1,000
$300 million, 3.850% Senior Notes due 2023 [Member]
Financing
Senior notes	298,943		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	1,000		1,200
$500 million, 3.550% Senior Notes due 2026 [Member]
Financing
Senior notes	496,470		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	2,900		3,100
$750 million, 3.600% Senior Notes due 2027 [Member]
Financing
Senior notes	744,172		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	4,800		5,100
$500 million, 4.350% Senior Notes due 2028 [Member]
Financing
Senior notes	495,222		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
$500 million, 3.900% Senior Notes due 2029 [Member]
Financing
Senior notes	496,012
Senior notes, face amount	$ 500,000	$ 500,000
Interest rate of senior notes	3.90%	3.90%
Senior notes, effective interest rate	3.901%
Senior notes, unamortized discount	$ 100
Senior notes, unamortized debt issuance costs	3,900
Revolving Credit Facility [Member]
Financing
Unsecured revolving credit facility	$ 156,000		$ 287,000
Unsecured revolving credit facility, weighted-average variable interest rate	4.229%

[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.