Debt (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term debt outstanding	$ 297,436	$ 512,970
Commercial Paper
Short-term Debt [Line Items]
Short-term debt outstanding	$ 297,400	$ 513,000
Weighted average interest rate	1.40%	0.90%
Average daily borrowings	$ 449,600	$ 422,900
Weighted average interest rates on average daily borrowings	1.30%	0.60%