Capital leases (Details)	12 Months Ended	0 Months Ended
	Dec. 31, 1985 Scherer Unit No. 2 item	Jun. 14, 2012 Lease renewal through December 31, 2027 item	Jun. 14, 2012 Lease renewal through December 31, 2031 item
Capital leases
Number of purchasers from which entity sold and leased back assets	4
Percentage of undivided ownership interest of purchasers in assets under lease	60.00%
Term of lease extension for which irrevocable notice of renewal is executed		14 years 6 months
Number of leases for which lease term is extended		3	1
Lease term			18 years