Equity-Indexed Compensation Plans (Details) - Long-Term Incentive Plan Awards shares in Millions	9 Months Ended
	Sep. 30, 2015 $ / shares shares
Outstanding (in units)
Outstanding at beginning of period (in units)	7.3
Granted (in units)	1.3
Vested (in units)	(2.1)
Cancelled or forfeited (in units)	(0.3)
Outstanding at end of period (in units)	6.2
Weighted Average Grant Date Fair Value per Unit
Outstanding at beginning of period (in dollars per unit) | $ / shares	$ 41.45
Granted (in dollars per unit) | $ / shares	38.91
Vested (in dollars per unit) | $ / shares	28.91
Cancelled or forfeited (in dollars per unit) | $ / shares	45.68
Outstanding at end of period (in dollars per unit) | $ / shares	$ 44.98
Units issued in connection with the settlement of vested awards, net of tax withholding (in units)	0.5
Units withheld for taxes (in units)	0.3
Vested awards settled in cash (in units)	1.3