Employee Incentive Plans Employee Incentive Plans, Weighted Average Grant Date Fair Value (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-Average Grant-Date Fair Value per Share	$ 86.92	$ 86.11	$ 72.09
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-Average Grant-Date Fair Value per Share	76.13	75.29	69.83
TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-Average Grant-Date Fair Value per Share	97.88	93.02	95.68
CP-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-Average Grant-Date Fair Value per Share	79.69	80.46	0.00
EVA-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Weighted-Average Grant-Date Fair Value per Share	$ 79.69	$ 0.00	$ 0.00