Debt (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Short-term debt outstanding	$ 356,951	$ 488,988
Commercial Paper
Short-term Debt [Line Items]
Short-term debt outstanding	$ 357,000	$ 489,000
Weighted average interest rates on short-term debt outstanding	0.30%	0.30%
Average daily borrowings	$ 407,900	$ 335,000
Weighted average interest rates on average daily borrowings	0.30%	0.20%