Debt (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Short-term Debt [Abstract]
Short-term Debt		$ 152,800	$ 190,300	$ 270,035
Commercial Paper, Maximum Maturity		67 days
Commercial Paper, Average Maturity		35 days
Debt Outstanding
Long-term Debt, Gross		$ 601,700	621,700	601,700
Unamortized Debt Issuance Expense		(6,668)	(7,963)	(7,282)
Long-term Debt		595,032	613,737	594,418
Long-term Debt, Current Maturities		24,987	0	24,973
Long-term debt, noncurrent		$ 570,045	$ 613,737	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		5.70%
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]	$ 708,322	$ 695,902	$ 667,168

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