Employee Benefit Plans - Expected long-term return by asset category (Details)	12 Months Ended
	Dec. 31, 2016
Expected Long Term Return [Abstract]
Expected long-term return	5.40%
Asset allocation [Abstract]
Asset allocation	100.00%
Growth assets
Expected Long Term Return [Abstract]
Expected long-term return	7.60%
Asset allocation [Abstract]
Asset allocation	61.00%
Hedging assets
Expected Long Term Return [Abstract]
Expected long-term return	2.00%
Asset allocation [Abstract]
Asset allocation	35.00%
Cash
Expected Long Term Return [Abstract]
Expected long-term return	0.30%
Asset allocation [Abstract]
Asset allocation	4.00%