Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Debt Disclosure [Abstract]
Interest expense	$ 33,318	$ 33,005
Interest income	(1,604)	(968)
Interest expense, net	31,714	$ 32,037
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	4,558		$ 122
Letters of Credit Outstanding, Amount	91,840
Short-term borrowings [Abstract]
Commercial Paper	401,300		151,000
Debt, Current	405,858		151,122
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Net	(13,145)		(13,687)
Long-term borrowings [Abstract]
Carrying amount of long-term debt	2,623,787		2,617,342
Long-term debt	2,610,642		2,603,655
Line of Credit Facility [Abstract]
Unsecured revolving credit facility, maximum borrowing capacity	$ 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 March 31, 2016 and had a coverage ratio of 12.3 to 1
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	$ 349,340		349,258
Debt instruments, maturity date	Mar. 15, 2018
Debt instrument, stated interest rate (in hundredths)	5.45%
Term of debt instrument (in years)	10 years
Note due 2020 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 334,783		328,592
Debt instruments, maturity date	Dec. 01, 2020
Debt instrument, stated interest rate (in hundredths)	2.125%
Term of debt instrument (in years)	7 years
Note due 2021 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 449,872		449,865
Debt instruments, maturity date	Mar. 01, 2021
Debt instrument, stated interest rate (in hundredths)	4.30%
Term of debt instrument (in years)	10 years
Note due 2025 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 397,028		396,951
Debt instruments, maturity date	Nov. 15, 2025
Debt instrument, stated interest rate (in hundredths)	3.15%
Term of debt instrument (in years)	10 years
Debentures due 2028 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 199,560		199,552
Debt instruments, maturity date	Jun. 01, 2028
Debt instrument, stated interest rate (in hundredths)	6.65%
Term of debt instrument (in years)	30 years
Debenture due 2035 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 296,884		296,844
Debt instruments, maturity date	Oct. 15, 2035
Debt instrument, stated interest rate (in hundredths)	5.375%
Term of debt instrument (in years)	30 years
Note due 2038 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 248,058		248,036
Debt instruments, maturity date	Mar. 15, 2038
Debt instrument, stated interest rate (in hundredths)	6.60%
Term of debt instrument (in years)	30 years
Note due 2041 [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 346,029		345,989
Debt instruments, maturity date	Mar. 01, 2041
Debt instrument, stated interest rate (in hundredths)	5.375%
Term of debt instrument (in years)	30 years
Other long term debt instruments [Member]
Long-term borrowings [Abstract]
Long-term debt	$ 2,233		$ 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, 2020