Employee Benefit Plans (Details) (USD $)	1 Months Ended	3 Months Ended			6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended			1 Months Ended	3 Months Ended			6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended			6 Months Ended		3 Months Ended		6 Months Ended
	Jan. 31, 2014 Pension Plans	Jun. 30, 2014 Pension Plans	Mar. 31, 2014 Pension Plans	Jun. 30, 2013 Pension Plans	Jun. 30, 2014 Pension Plans	Jun. 30, 2013 Pension Plans	Jun. 30, 2014 Postretirement Benefit Plans	Jun. 30, 2013 Postretirement Benefit Plans	Jun. 30, 2014 Postretirement Benefit Plans	Jun. 30, 2013 Postretirement Benefit Plans	Mar. 31, 2014 Supplemental Employee Retirement Plan	Mar. 31, 2013 Supplemental Employee Retirement Plan	Jun. 30, 2014 Alabama Gas Corporation pension_plan	Jan. 31, 2014 Alabama Gas Corporation Pension Plans	Jun. 30, 2014 Alabama Gas Corporation Pension Plans	Mar. 31, 2014 Alabama Gas Corporation Pension Plans	Jun. 30, 2013 Alabama Gas Corporation Pension Plans	Jun. 30, 2014 Alabama Gas Corporation Pension Plans	Jun. 30, 2013 Alabama Gas Corporation Pension Plans	Jun. 30, 2014 Alabama Gas Corporation Postretirement Benefit Plans	Jun. 30, 2013 Alabama Gas Corporation Postretirement Benefit Plans	Jun. 30, 2014 Alabama Gas Corporation Postretirement Benefit Plans	Jun. 30, 2013 Alabama Gas Corporation Postretirement Benefit Plans	Mar. 31, 2013 Alabama Gas Corporation Supplemental Employee Retirement Plan	Jun. 30, 2014 Alabama Gas Corporation Pension Plans	Jun. 30, 2013 Alabama Gas Corporation Pension Plans	Jun. 30, 2014 Alabama Gas Corporation Pension Plans	Jun. 30, 2013 Alabama Gas Corporation Pension Plans	Jun. 30, 2014 Alabama Gas Corporation Postretirement Benefit Plans	Jun. 30, 2013 Alabama Gas Corporation Postretirement Benefit Plans	Jun. 30, 2014 Alabama Gas Corporation Postretirement Benefit Plans	Jun. 30, 2013 Alabama Gas Corporation Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Number of defined benefit non-contributory pension plans and certain nonqualified supplemental pension plans													2
Components of net periodic benefit cost:
Service cost		$ 1,621,000		$ 1,327,000	$ 3,256,000	$ 2,654,000	$ 59,000	$ 112,000	$ 119,000	$ 223,000					$ 1,696,000		$ 2,176,000	$ 3,393,000	$ 4,352,000	$ 132,000	$ 314,000	$ 265,000	$ 629,000
Interest cost		1,404,000		1,088,000	2,817,000	2,176,000	224,000	176,000	450,000	351,000					1,439,000		1,612,000	2,878,000	3,225,000	614,000	664,000	1,227,000	1,328,000
Expected long-term return on assets		(1,449,000)		(1,323,000)	(2,909,000)	(2,645,000)	(383,000)	(214,000)	(770,000)	(426,000)					(1,679,000)		(2,315,000)	(3,359,000)	(4,632,000)	(1,172,000)	(994,000)	(2,343,000)	(1,989,000)
Actuarial loss		1,407,000		1,823,000	2,819,000	3,657,000	(209,000)	0	(421,000)	0					1,110,000		1,937,000	2,220,000	3,878,000	(388,000)	0	(777,000)	0
Prior service cost amortization		57,000		62,000	114,000	123,000									71,000		72,000	143,000	144,000
Settlement charge		115,000		0	3,673,000	144,000
Transition amortization							15,000	63,000	30,000	126,000										17,000	251,000	34,000	502,000
Net periodic expense		3,155,000		2,977,000	9,770,000	6,109,000	(294,000)	137,000	(592,000)	274,000					2,637,000		3,482,000	5,275,000	6,967,000	(797,000)	235,000	(1,594,000)	470,000		2,637,000	3,482,000	5,275,000	6,967,000	(797,000)	235,000	(1,594,000)	470,000
Estimated required future employer contributions in current fiscal year		1,600,000			1,600,000										2,900,000			2,900,000
Contributions by employer in current fiscal year	1,600,000													1,400,000
Benefit payments made in current fiscal year		33,000			913,000
Estimated future benefit payments in current fiscal year		5,200,000			5,200,000
Settlement charges		700,000									400,000	500,000
Settlement charges expensed		400,000	6,900,000									100,000
Settlements recognized as a pension asset															$ 300,000	$ 10,200,000								$ 400,000