Pension Plan and Employee Benefits, Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 440,876	$ 452,991
Amounts recognized in other comprehensive income [Abstract]
Net actuarial loss (gain) occurring during year	3,128	74,242
Net actuarial loss amortized during year	13,828	6,743
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)	8,935
Prior service cost (credit)	(71)
Amounts in accumulated other comprehensive income [Abstract]
Net actuarial loss (gain)	150,620	161,320
Prior service cost (credit)	(345)	(417)
Other Benefits [Member]
Amounts recognized in other comprehensive income [Abstract]
Net actuarial loss (gain) occurring during year	(1,039)	1,228
Net actuarial loss amortized during year	866	670
Transition obligation (asset) amortized during year	0	16
Prior service cost (credit) amortized during year	119	119
Amounts in accumulated other comprehensive income to be recognized in next fiscal year [Abstract]
Net actuarial loss (gain)	666
Prior service cost (credit)	119
Amounts in accumulated other comprehensive income [Abstract]
Net actuarial loss (gain)	8,805	10,710
Prior service cost (credit)	363	482
SERP Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	65,840	67,126
Amounts recognized in other comprehensive income [Abstract]
Net actuarial loss (gain) occurring during year	(4,487)	13,436
Net actuarial loss amortized during year	2,973	1,876
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)	2,033
Prior service cost (credit)	59
Amounts in accumulated other comprehensive income [Abstract]
Net actuarial loss (gain)	23,763	31,224
Prior service cost (credit)	$ 120	$ 173