Employee Benefit Plans (Benefit Obligations) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Benefit obligation:
Termination benefit charge	$ 2,500
Amounts recognized in balance sheet
Noncurrent assets	0	$ 3,881	$ 0
Noncurrent liabilities	(15,935)	0	(15,935)
Amounts recognized to accumulated other comprehensive income:
Total accumulated other comprehensive income (loss)	22,870	(263)	22,870
Nonqualified Supplemental Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Reclassification of assets		3,300
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	107,669	15,729	107,669
Benefit obligation:
Balance at beginning of period		107,669	266,294
Service cost		0	8,329
Interest cost		816	5,325
Actuarial (gain) loss		(683)	9,078
Plan amendments		0	0
Curtailment gain		0	(8,496)
Transfer in connection with the sale of Alagasco		0	(124,783)
Termination benefit charge		0	2,477	$ 0
Retiree drug subsidy program		0	0
Benefits paid		(92,073)	(50,555)
Balance at end of period	107,669	15,729	107,669	266,294
Plan assets:
Fair value of plan assets at beginning of period		67,542	193,457
Actual return (loss) on plan assets		(289)	5,359
Employer contributions		24,847	19,164
Transfer in connection with the sale of Alagasco		0	(99,883)
Benefits paid		(92,073)	(50,555)
Fair value of plan assets at end of period	67,542	27	67,542	193,457
Funded status of plans	(40,127)	(15,702)	(40,127)
Amounts recognized in balance sheet
Noncurrent assets	0	0	0
Current liabilities	(24,626)	(15,702)	(24,626)
Noncurrent liabilities	(15,501)	0	(15,501)
Net asset (liability) recognized	(40,127)	(15,702)	(40,127)
Amounts recognized to accumulated other comprehensive income:
Prior service credit, net of taxes	0	0	0
Net actuarial loss, net of taxes	22,246	2,179	22,246
Total accumulated other comprehensive income (loss)	22,246	2,179	22,246
Postretirement Benefits
Benefit obligation:
Balance at beginning of period		11,127	33,224
Service cost		392	262
Interest cost		466	716
Actuarial (gain) loss		(1,185)	6,385
Plan amendments		(4,071)	0
Curtailment gain		0	0
Transfer in connection with the sale of Alagasco		0	(28,648)
Termination benefit charge		0	0
Retiree drug subsidy program		0	48
Benefits paid		(241)	(860)
Balance at end of period	11,127	6,488	11,127	33,224
Plan assets:
Fair value of plan assets at beginning of period		10,693	55,459
Actual return (loss) on plan assets		(83)	(331)
Employer contributions		0	21
Transfer in connection with the sale of Alagasco		0	(43,596)
Benefits paid		(241)	(860)
Fair value of plan assets at end of period	10,693	10,369	10,693	$ 55,459
Funded status of plans	(434)	3,881	(434)
Amounts recognized in balance sheet
Noncurrent assets	0	3,881	0
Current liabilities	0	0	0
Noncurrent liabilities	(434)	0	(434)
Net asset (liability) recognized	(434)	3,881	(434)
Amounts recognized to accumulated other comprehensive income:
Prior service credit, net of taxes	0	(2,646)	0
Net actuarial loss, net of taxes	624	205	624
Total accumulated other comprehensive income (loss)	$ 624	$ (2,441)	$ 624