Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Debt Disclosure [Abstract]
Interest expense	$ 33,779	$ 33,053	$ 67,097	$ 66,058
Interest income	(1,622)	(1,065)	(3,226)	(2,033)
Interest expense, net	32,157	$ 31,988	63,871	$ 64,025
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	7,157		7,157		$ 122
Letters of Credit Outstanding, Amount	100,319		100,319
Short-term borrowings [Abstract]
Commercial Paper	337,000		337,000		151,000
Debt, Current	344,157		344,157		151,122
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Net	(12,781)		(12,781)		(13,687)
Long-term borrowings [Abstract]
Carrying amount of long-term debt	2,619,847		2,619,847		2,617,342
Long-term debt	2,607,066		2,607,066		2,603,655
Line of Credit Facility [Abstract]
Unsecured revolving credit facility, maximum borrowing capacity	1,000,000		$ 1,000,000
Line of Credit Facility, Covenant Compliance			The Company was in compliance with its revolving credit and other long-term debt covenants at June 30, 2016 and had a coverage ratio of 9.7 to 1.0.
Credit Agreement [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date			Nov. 10, 2020
Note due 2018 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 349,423		$ 349,423		349,258
Debt instruments, maturity date			Mar. 15, 2018
Debt instrument, stated interest rate (in hundredths)	5.45%		5.45%
Term of debt instrument (in years)			10 years
Note due 2020 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 330,303		$ 330,303		328,592
Debt instruments, maturity date			Dec. 01, 2020
Debt instrument, stated interest rate (in hundredths)	2.125%		2.125%
Term of debt instrument (in years)			7 years
Note due 2021 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 449,878		$ 449,878		449,865
Debt instruments, maturity date			Mar. 01, 2021
Debt instrument, stated interest rate (in hundredths)	4.30%		4.30%
Term of debt instrument (in years)			10 years
Note due 2025 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 397,105		$ 397,105		396,951
Debt instruments, maturity date			Nov. 15, 2025
Debt instrument, stated interest rate (in hundredths)	3.15%		3.15%
Term of debt instrument (in years)			10 years
Debentures due 2028 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 199,569		$ 199,569		199,552
Debt instruments, maturity date			Jun. 01, 2028
Debt instrument, stated interest rate (in hundredths)	6.65%		6.65%
Term of debt instrument (in years)			30 years
Debenture due 2035 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 296,924		$ 296,924		296,844
Debt instruments, maturity date			Oct. 15, 2035
Debt instrument, stated interest rate (in hundredths)	5.375%		5.375%
Term of debt instrument (in years)			30 years
Note due 2038 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 248,080		$ 248,080		248,036
Debt instruments, maturity date			Mar. 15, 2038
Debt instrument, stated interest rate (in hundredths)	6.60%		6.60%
Term of debt instrument (in years)			30 years
Note due 2041 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 346,069		$ 346,069		345,989
Debt instruments, maturity date			Mar. 01, 2041
Debt instrument, stated interest rate (in hundredths)	5.375%		5.375%
Term of debt instrument (in years)			30 years
Other long term debt instruments [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 2,496		$ 2,496		$ 2,255
Minimum [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date			Dec. 31, 2016
Maximum [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date			Dec. 31, 2023