Employee Benefit Plans - Expected long-term return by asset category (Details)	12 Months Ended
	Dec. 31, 2017
Expected Long Term Return [Abstract]
Expected long-term return	5.50%
Asset allocation [Abstract]
Asset allocation	100.00%
Growth assets
Expected Long Term Return [Abstract]
Expected long-term return	7.70%
Asset allocation [Abstract]
Asset allocation	64.00%
Hedging assets
Expected Long Term Return [Abstract]
Expected long-term return	1.80%
Asset allocation [Abstract]
Asset allocation	34.00%
Cash
Expected Long Term Return [Abstract]
Expected long-term return	0.50%
Asset allocation [Abstract]
Asset allocation	2.00%