Pension Plan and Employee Benefits, Fair Value of Pension Plan Assets (Details) - Pension Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 392,949	$ 361,852
Investments Measured at Net Asset Value	48,103	38,886
Interest Accrual	3,037	2,977
Total net assets	444,089	403,715
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,825	6,817
Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17,655	19,311
Obligations of Government, Government Agencies, and state and local governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,852	47,543
Mutual funds, Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58,617	52,663
Mutual funds, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36,970	31,191
Real estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19,195	18,668
Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	204,835	185,659
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	113,242	103,165
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 [Member] | Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17,655	19,311
Level 1 [Member] | Obligations of Government, Government Agencies, and state and local governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 [Member] | Mutual funds, Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58,617	52,663
Level 1 [Member] | Mutual funds, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36,970	31,191
Level 1 [Member] | Real estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 [Member] | Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	260,512	240,019
Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,825	6,817
Level 2 [Member] | Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 [Member] | Obligations of Government, Government Agencies, and state and local governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,852	47,543
Level 2 [Member] | Mutual funds, Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 [Member] | Mutual funds, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 [Member] | Real estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 [Member] | Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	204,835	185,659
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19,195	18,668
Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 [Member] | Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 [Member] | Obligations of Government, Government Agencies, and state and local governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 [Member] | Mutual funds, Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 [Member] | Mutual funds, International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 [Member] | Real estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19,195	18,668
Level 3 [Member] | Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0