Debt (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Short-term debt outstanding	$ 386,952	$ 581,700
Commercial Paper
Short-term Debt [Line Items]
Short-term debt outstanding	$ 387,000	$ 581,700
Weighted average interest rates on short-term debt outstanding	0.60%	0.50%
Average daily borrowings	$ 425,600	$ 387,900
Weighted average interest rates on average daily borrowings	0.60%	0.30%