Retirement Benefits - Amounts Recognized (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 11,627	$ 10,636	$ 11,454
Accumulated benefit obligation	11,604	10,557
Fair value of plan assets	6,289	5,960	7,138
Defined Benefit Plan, Amounts Recognized in Statement of Financial Position Consist of: [Abstract]
Noncurrent assets	0	0
Current liabilities	(308)	(332)
Noncurrent liabilities	(5,030)	(4,344)
Net assets (liabilities)	(5,338)	(4,676)
Parent Company [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	(197)	(2,564)
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	0	0
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax, Total	(197)	(2,564)
Subsidiaries [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	(364)	(3,857)
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	0	0
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax, Total	(364)	(3,857)
Nonqualified Plan | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	63,000	55,708	55,754
Accumulated benefit obligation	59,988	51,031
Fair value of plan assets	0	0	0
Defined Benefit Plan, Amounts Recognized in Statement of Financial Position Consist of: [Abstract]
Noncurrent assets	0	0
Current liabilities	(22,604)	(6,249)
Noncurrent liabilities	(40,396)	(49,459)
Net assets (liabilities)	(63,000)	(55,708)
Nonqualified Plan | Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	15,003	9,612
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	1,276	1,607
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax, Total	16,279	11,219
Nonqualified Plan | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	16,473	11,450
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	1,276	1,609
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax, Total	17,749	13,059
Qualified Plan | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	774,305	677,643	700,481
Accumulated benefit obligation	762,838	668,469
Fair value of plan assets	753,042	640,242	$ 704,360
Defined Benefit Plan, Amounts Recognized in Statement of Financial Position Consist of: [Abstract]
Noncurrent assets	0	0
Current liabilities	0	0
Noncurrent liabilities	(21,263)	(37,401)
Net assets (liabilities)	(21,263)	(37,401)
Qualified Plan | Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	94,319	94,929
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	(3,884)	(5,863)
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax, Total	90,435	89,066
Qualified Plan | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan, Amounts Recognized in Accumulated Other Comprehensive Income Consist of: [Abstract]
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	217,502	229,819
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	(3,086)	(4,659)
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax, Total	$ 214,416	$ 225,160