Debt (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term debt outstanding	$ 607,941	$ 512,970
Commercial Paper
Short-term Debt [Line Items]
Short-term debt outstanding	$ 607,900	$ 513,000
Weighted average interest rates on short-term debt outstanding	1.10%	0.90%
Average daily borrowings	$ 532,200	$ 422,900
Weighted average interest rates on average daily borrowings	0.90%	0.60%