Employee Benefit Plans (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014
Pension and Other Postretirement Benefits
Percentage workforce of the entity represented by UMWA	49.50%
Pension Act surcharge (percentage)	5.00%
Pension Act surcharge starting July 1, 2015 (percentage)	10.00%
Number of multiemployer benefit plans	2
UMWA 1974 Pension Plan
Pension and Other Postretirement Benefits
Required contribution per hour worked by employees under collective bargaining agreement (in dollars per hour)	5.50
Percentage required for getting listed under Form 5500 filed (more than)			5.00%
Required funded percentage under Pension Protection Act (less than)	80.00%
Maximum funded percentage for specified period to deem plan as endangered or seriously endangered (less than)	80.00%
Maximum number of years to reach a projected funding deficiency in addition to specified funded percentage to deem plan as endangered or seriously endangered	7 years
Minimum funded percentage for specified period to deem plan as critical (less than)	65.00%
Funded percentage of multi-employer pension plan as certified by the actuary (less than)	65.00%
Required contribution per hour worked by employees under collective bargaining agreement, including surcharge (in dollars per hour)	5.78
Required contribution per hour worked by employees under collective bargaining agreement, including surcharge starting July 1, 2015 (in dollars per hour)	6.05
Minimum improvement in funded plan status under the Funding Improvement Plan (as a percent)	20.00%
Period for a specified improvement in funded plan status under the Funding Improvement Plan	15 years
Number of plan years used in calculating employer's share of the unfunded vested benefits	5 years
Unfunded vested benefits				$ 4,300,000,000
Percentage of hours worked compared during the previous five plan years to the total hours worked by all plan participants	15.00%
Entity's combined withdrawal liability	661,200,000
Contributions to the plans	17,854,000	19,670,000	20,948,000
UMWA 1993 Benefit Plan
Pension and Other Postretirement Benefits
Contributions to the plans	3,600,000	3,900,000	4,200,000
Contribution per hour worked by employees under 2011 labor agreement (in dollars per hour)	1.10	1.10	1.10
UMWA 2012 Retiree Bonus Account Trust and Plan
Pension and Other Postretirement Benefits
Contributions to the plans	5,100,000	5,500,000	5,700,000
Contribution per hour worked by employees under 2011 labor agreement (in dollars per hour)	1.56
One-time retiree bonus payment	580
One-time disabled and other retiree bonus payment	$ 455