Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount	$ 357,126	$ 6,950
Debt Instrument, Unamortized Discount	18,200	18,800
Letters of Credit Outstanding, Amount	136,500
Short-term borrowings [Abstract]
Commercial Paper	391,300	407,600
Debt, Current	748,426	414,550
Deferred Finance Costs, Own-share Lending Arrangement, Issuance Costs, Net	(16,300)	(16,500)
Long-term borrowings [Abstract]
Long-term debt	3,238,763	3,207,777
Long-term Debt, Current Maturities	(350,801)	(1,140)
Carrying amount of long-term debt	2,887,962	3,206,637
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 all covenants in the Credit Agreement and other long-term debt covenants at March 31, 2017 and had a coverage ratio of 10.1 to 1.0.
Credit Agreement [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date	Nov. 10, 2020
Note due 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 350,000
Long-term borrowings [Abstract]
Long-term debt	$ 349,623	349,502
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]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 323,957
Long-term borrowings [Abstract]
Long-term debt	$ 322,207	311,851
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]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 450,000
Long-term borrowings [Abstract]
Long-term debt	$ 448,554	448,458
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]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 400,000
Long-term borrowings [Abstract]
Long-term debt	$ 394,189	394,042
Debt instruments, maturity date	Nov. 15, 2025
Debt instrument, stated interest rate (in hundredths)	3.15%
Term of debt instrument (in years)	10 years
Note due 2026 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 647,913
Long-term borrowings [Abstract]
Long-term debt	637,200	616,893
Debentures due 2028 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	200,000
Long-term borrowings [Abstract]
Long-term debt	$ 198,879	198,830
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]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 300,000
Long-term borrowings [Abstract]
Long-term debt	$ 295,374	295,316
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]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 250,000
Long-term borrowings [Abstract]
Long-term debt	$ 247,628	247,593
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]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 350,000
Long-term borrowings [Abstract]
Long-term debt	$ 343,393	343,323
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	$ 1,716	$ 1,969
Minimum [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date	Dec. 31, 2017
Maximum [Member]
Line of Credit Facility [Abstract]
Line of Credit Facility, Expiration Date	Dec. 31, 2039