Pension Plan and Employee Benefits, Amounts Recognized in Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 452,991	$ 358,128
Amounts recognized in other comprehensive income [Abstract]
Net actuarial loss (gain) occurring during year	74,242	(49,978)
Prior service cost occurring during year	0	0
Net actuarial loss amortized during year	6,743	13,218
Transition obligation (asset) amortized during year	0	0
Prior service cost (credit) amortized during year	(71)	(71)
Amounts in accumulated other comprehensive income to be recognized in next fiscal year [Abstract]
Net actuarial loss (gain)	13,491
Transition obligation (asset)	0
Prior service cost (credit)	(71)
Amounts in accumulated other comprehensive income [Abstract]
Net actuarial loss (gain)	161,320	93,821
Transition obligation (asset)	0	0
Prior service cost (credit)	(417)	(488)
Other Benefits [Member]
Amounts recognized in other comprehensive income [Abstract]
Net actuarial loss (gain) occurring during year	1,228	(1,768)
Prior service cost occurring during year	0	601
Net actuarial loss amortized during year	670	1,131
Transition obligation (asset) amortized during year	16	20
Prior service cost (credit) amortized during year	119	0
Amounts in accumulated other comprehensive income to be recognized in next fiscal year [Abstract]
Net actuarial loss (gain)	865
Transition obligation (asset)	0
Prior service cost (credit)	119
Amounts in accumulated other comprehensive income [Abstract]
Net actuarial loss (gain)	10,710	10,703
Transition obligation (asset)	0	16
Prior service cost (credit)	482	601
SERP Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	67,126	53,046
Amounts recognized in other comprehensive income [Abstract]
Net actuarial loss (gain) occurring during year	13,436	(3,477)
Net actuarial loss amortized during year	1,876	2,305
Prior service cost (credit) amortized during year	54	54
Amounts in accumulated other comprehensive income to be recognized in next fiscal year [Abstract]
Net actuarial loss (gain)	2,919
Prior service cost (credit)	54
Amounts in accumulated other comprehensive income [Abstract]
Net actuarial loss (gain)	31,224	19,663
Prior service cost (credit)	$ 173	$ 227

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

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The pretax net amount of gains and losses that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases or decreases in other comprehensive income as they arise. Gains and losses are due to changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan.

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Amount related to the pretax cost of benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost.

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Before tax amount of net transition assets (obligations) which has yet to be recognized as a component of net periodic benefit cost.

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Amount included in accumulated other comprehensive income (AOCI) related to transition obligation (asset) expected to be recognized as components of net periodic benefit cost (credit) for the fiscal year that follows the most recent annual statement of financial position.

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Amount after tax of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans.

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Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.

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