Employee Benefit Plans - Expected long-term return by asset category (Details)	12 Months Ended
	Dec. 31, 2018
Expected Long Term Return [Abstract]
Expected long-term return	6.40%
Asset allocation [Abstract]
Asset allocation	100.00%
Growth assets
Expected Long Term Return [Abstract]
Expected long-term return	8.60%
Asset allocation [Abstract]
Asset allocation	68.00%
Hedging assets
Expected Long Term Return [Abstract]
Expected long-term return	1.80%
Asset allocation [Abstract]
Asset allocation	30.00%
Cash
Expected Long Term Return [Abstract]
Expected long-term return	0.80%
Asset allocation [Abstract]
Asset allocation	2.00%