Share-Based Payments Non-vested (Details) - $ / shares shares in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
RSUs and DSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested awards outstanding at the beginning of the period (in shares)	1.0
Granted (in shares)	0.4
Vested (in shares)	(0.3)
Forfeited (in shares)	0.0
Non-vested awards outstanding at the end of the period (in shares)	1.1
Non-vested, weighted-average grant date fair value at the beginning of the period (in dollars per unit)	$ 95.80
Granted, weighted-average grant date fair value (in dollars per unit)	74.20
Vested, weighted-average grant date fair value (in dollars per unit)	91.55
Forfeited, weighted-average grant date fair value (in dollars per unit)	0.00
Non-vested, weighted-average grant date fair value at the end of the period (in dollars per unit)	$ 89.93
PSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested awards outstanding at the beginning of the period (in shares)	0.4
Granted (in shares)	0.2
Vested (in shares)	(0.1)
Forfeited (in shares)	0.0
Non-vested awards outstanding at the end of the period (in shares)	0.5
Non-vested, weighted-average grant date fair value at the beginning of the period (in dollars per unit)	$ 89.57
Granted, weighted-average grant date fair value (in dollars per unit)	78.30	$ 97.13
Vested, weighted-average grant date fair value (in dollars per unit)	75.10
Forfeited, weighted-average grant date fair value (in dollars per unit)	0.00
Non-vested, weighted-average grant date fair value at the end of the period (in dollars per unit)	86.95
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Intrinsic Value, Amount Per Share	$ 86.76