Employee Benefit Plans - Expected long-term return by asset category (Details)	12 Months Ended
	Dec. 31, 2015
Expected Long Term Return [Abstract]
Expected long-term return	6.00%
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	68.00%
Hedging assets
Expected Long Term Return [Abstract]
Expected long-term return	2.60%
Asset allocation [Abstract]
Asset allocation	28.00%
Target return assets
Expected Long Term Return [Abstract]
Expected long-term return	7.10%
Asset allocation [Abstract]
Asset allocation	1.00%
Cash
Expected Long Term Return [Abstract]
Expected long-term return	0.50%
Asset allocation [Abstract]
Asset allocation	3.00%