Employee Benefit Plans (Details) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined Benefit Plan Disclosure
Number of defined benefit non-contributory pension plans and certain nonqualified supplemental pension plans		2		2
Components of net periodic benefit cost:
Settlement charges		$ 500,000
Settlement charges expensed		100,000
Pension Plans
Components of net periodic benefit cost:
Service cost		3,602,000	2,632,000	7,204,000	5,264,000
Interest cost		2,718,000	2,700,000	5,436,000	5,400,000
Expected long-term return on assets		(3,713,000)	(3,563,000)	(7,426,000)	(7,126,000)
Actuarial loss		3,675,000	2,099,000	7,365,000	4,198,000
Prior service cost amortization		122,000	129,000	245,000	259,000
Settlement charge		0	0	144,000	0
Net periodic expense		6,404,000	3,997,000	12,968,000	7,995,000
Estimated required future employer contributions in current fiscal year		0		0
Contributions by employer in current fiscal year	9,000,000
Estimated discretionary future employer contributions in current fiscal year		0		0
Benefit payments made in current fiscal year		0		1,000,000
Estimated future benefit payments in current fiscal year				200,000
Postretirement Benefit Plans
Components of net periodic benefit cost:
Service cost		444,000	463,000	888,000	927,000
Interest cost		869,000	1,062,000	1,737,000	2,124,000
Expected long-term return on assets		(1,242,000)	(1,109,000)	(2,484,000)	(2,219,000)
Actuarial loss		0	9,000	0	18,000
Transition amortization		324,000	479,000	648,000	959,000
Net periodic expense		395,000	904,000	789,000	1,809,000
Contributions by employer in current fiscal year		400,000		800,000
Estimated future employer contributions in current fiscal year				800,000
Alabama Gas Corporation
Components of net periodic benefit cost:
Settlements recognized as a pension asset		$ 400,000