Employee Benefits, Defined Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 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
VEBAs | Maximum
VEBA Postretirement Medical Obligations
Administrative expenses of the VEBAs	0.3	$ 0.3
Union VEBA
VEBA Postretirement Medical Obligations
Percent allocation of total contribution between VEBAs		85.50%
Accrued Veba contingent contribution - total	47.0	$ 47.0
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
Salaried VEBA
VEBA Postretirement Medical Obligations
Percent allocation of total contribution between VEBAs		14.50%
Variable cash contribution | VEBAs
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs			$ 13.7	$ 16.0
Variable cash contribution | VEBAs | Maximum
VEBA Postretirement Medical Obligations
Variable cash contribution obligation to VEBAs	$ 20.0	$ 20.0
Variable cash contribution | VEBAs | Annual Cash Flows up to $20 Million
VEBA Postretirement Medical Obligations
Postretirement medical plan contribution obligation percentage		10.00%
Variable cash contribution | VEBAs | Annual Cash Flows in Excess of $20 Million
VEBA Postretirement Medical Obligations
Postretirement medical plan contribution obligation percentage		20.00%