Employee Benefits, Defined Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs	$ 0.0		$ 0.0	$ 13.7
VEBAs
VEBA Postretirement Medical Obligations
Cash flow in determining VEBA obligation			20.0
Accrued annual contribution to VEBAs				13.7	$ 16.0
VEBAs | Maximum
VEBA Postretirement Medical Obligations
Administrative expenses of the VEBAs	0.3		0.3
Variable cash contribution obligation to VEBAs	20.0		$ 20.0
VEBAs | Annual Cash Flows up to $20 Million
VEBA Postretirement Medical Obligations
Postretirement medical plan contribution obligation percentage			10.00%
VEBAs | Annual Cash Flows in Excess of $20 Million
VEBA Postretirement Medical Obligations
Postretirement medical plan contribution obligation percentage			20.00%
Union VEBA
VEBA Postretirement Medical Obligations
Percent allocation of total contribution between VEBAs			85.50%
Accrued Veba contingent contribution - total	44.1		$ 44.1
Defined Benefit Plan, Benefit Obligation				391.5
Defined Benefit Plan, Fair Value of Plan Assets				$ 731.6
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments		$ 307.8
Income Tax Expense (Benefit)	$ 184.4	$ 184.4
Salaried VEBA
VEBA Postretirement Medical Obligations
Percent allocation of total contribution between VEBAs			14.50%