Retirement Benefits - Amounts Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	$ 10,636	$ 11,454	$ 11,194
Defined Benefit Plan, Accumulated Benefit Obligation	10,557	11,367
Defined Benefit Plan, Plan Assets, Amount	5,960	7,138	7,200
Defined Benefit Plan, Amounts Recognized in Statement of Financial Position Consist of: [Abstract]
Noncurrent assets	0	0
Current liabilities	(332)	(317)
Noncurrent liabilities	(4,344)	(3,999)
Net assets (liabilities)	(4,676)	(4,316)
Other changes (pre-tax) in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	488	403
Amortization of net (loss) gain	335	401
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	0
Amortization of prior service (cost) credit	0	0
Total change in other comprehensive income for year	823	804
Parent Company [Member] | Other Benefits
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Net loss (gain)	(2,564)	(3,386)
Prior service cost (credit)	0	0
Total	(2,564)	(3,386)
Subsidiaries [Member] | Other Benefits
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Net loss (gain)	(3,857)	(4,901)
Prior service cost (credit)	0	0
Total	(3,857)	(4,901)
Other changes (pre-tax) in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	488	452
Amortization of net (loss) gain	556	641
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	0
Amortization of prior service (cost) credit	0	0
Total change in other comprehensive income for year	1,044	1,093
Nonqualified Plan | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	55,708	55,754	51,734
Defined Benefit Plan, Accumulated Benefit Obligation	51,031	52,681
Defined Benefit Plan, Plan Assets, Amount	0	0	0
Defined Benefit Plan, Amounts Recognized in Statement of Financial Position Consist of: [Abstract]
Noncurrent assets	0	0
Current liabilities	(6,249)	(5,486)
Noncurrent liabilities	(49,459)	(50,268)
Net assets (liabilities)	(55,708)	(55,754)
Other changes (pre-tax) in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	1,122	2,722
Amortization of net (loss) gain	(1,580)	(1,076)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	(619)	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	1,446	0
Amortization of prior service (cost) credit	(42)	(42)
Total change in other comprehensive income for year	327	1,604
Nonqualified Plan | Parent Company [Member] | Qualified Pension Benefits
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Net loss (gain)	9,612	10,689
Prior service cost (credit)	1,607	204
Total	11,219	10,893
Nonqualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Net loss (gain)	11,450	13,134
Prior service cost (credit)	1,609	208
Total	13,059	13,342
Other changes (pre-tax) in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	1,122	2,722
Amortization of net (loss) gain	(2,069)	(1,565)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	(737)	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	1,446	0
Amortization of prior service (cost) credit	(44)	(44)
Total change in other comprehensive income for year	(282)	1,113
Qualified Plan | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation	677,643	700,481	652,607
Defined Benefit Plan, Accumulated Benefit Obligation	668,469	688,908
Defined Benefit Plan, Plan Assets, Amount	640,242	704,360	$ 620,260
Defined Benefit Plan, Amounts Recognized in Statement of Financial Position Consist of: [Abstract]
Noncurrent assets	0	3,879
Current liabilities	0	0
Noncurrent liabilities	(37,401)	0
Net assets (liabilities)	(37,401)	3,879
Other changes (pre-tax) in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	59,422	(18,896)
Amortization of net (loss) gain	(2,187)	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	0
Amortization of prior service (cost) credit	1,980	1,980
Total change in other comprehensive income for year	59,215	(16,916)
Qualified Plan | Parent Company [Member] | Qualified Pension Benefits
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Net loss (gain)	94,929	37,693
Prior service cost (credit)	(5,863)	(7,843)
Total	89,066	29,850
Qualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Net loss (gain)	229,819	185,277
Prior service cost (credit)	(4,659)	(6,232)
Total	225,160	179,045
Other changes (pre-tax) in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	59,460	(18,817)
Amortization of net (loss) gain	(14,917)	(13,048)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	0
Amortization of prior service (cost) credit	1,573	1,573
Total change in other comprehensive income for year	46,116	$ (30,292)
Scenario, Plan [Member] | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Net Gain (Loss), Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	(500)
Scenario, Plan [Member] | Subsidiaries [Member] | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Net Gain (Loss), Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	(400)
Scenario, Plan [Member] | Nonqualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Net Gain (Loss), Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	(1,700)
Pension and Other Postretirement Benefit Plans, Prior Service Cost, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	(300)
Scenario, Plan [Member] | Qualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Pension and Other Postretirement Benefit Plans, Net Gain (Loss), Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	$ (12,700)