Incentive Stock Plans - Schedule of Assumptions used for Performance Shares (Details) - Performance Shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	23.30%	25.40%	21.90%
Risk-free rate	1.50%	0.90%	0.90%