Employee Benefits, Defined Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs	$ 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
Union VEBA
VEBA Postretirement Medical Obligations
Percent allocation of total contribution between VEBAs	85.50%
Accrued Veba contingent contribution - total	45.4
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
Variable cash contribution | VEBAs | Maximum
VEBA Postretirement Medical Obligations
Variable cash contribution obligation to VEBAs	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%
Variable cash contribution | Union VEBA
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs		11.7
Variable cash contribution | Salaried VEBA
VEBA Postretirement Medical Obligations
Accrued annual contribution to VEBAs		$ 2.0