Debt (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Short-term Debt [Abstract]
Short-term Debt		$ 164,900	$ 156,200	$ 270,035
Repayments of Bank Debt		$ 50,000
Commercial Paper, Maximum Maturity		43 days
Commercial Paper, Average Maturity		21 days
Maturities of Long-term Debt [Abstract]
Long-term Debt, Current Maturities		$ 24,980	39,994	24,973
Debt Outstanding
Long-term Debt, Gross		601,700	661,700	601,700
Unamortized Debt Issuance Expense		(6,975)	(8,289)	(7,282)
Long-term Debt		594,725	653,411	594,418
Long-term Debt, Current Maturities		24,980	39,994	24,973
Long-term debt, noncurrent		$ 569,745	$ 613,417	569,445
Repayments of Long-term Debt [Abstract]
Debt Instrument, Maturity Date Range, Start		Dec. 15, 2016	Oct. 31, 2042
Long-term Debt, Weighted Average Interest Rate		5.698%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum		3.176%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum		9.05%
Liabilities, Fair Value Disclosure [Abstract]
Debt Instrument, Fair Value Disclosure	[1]	$ 686,159	$ 762,554	$ 667,168

[1] Estimated fair value does not include unamortized debt issuance costs.