Retirement Benefits - Change in Net Benefit Obligation and Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Other Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	$ 10,636	$ 11,454
Amendments	(9,049)	0
Service cost	61	69
Interest cost	410	444
Curtailment Loss / (Gain)	(7,486)	0
Actuarial loss (gain)	287	379
Benefits paid	982	1,037
Medicare part D subsidy received	226	85
Administrative expense	0	0
Benefit obligation at end of period	11,627	10,636
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	5,960	7,138
Actual return on plan assets	1,006	(395)
Employer contribution	305	254
Benefits paid	(982)	(1,037)
Administrative expense	0	0
Fair value of plan assets at end of period	6,289	5,960
Funded status at end of period	(5,338)	(4,676)
Nonqualified Plan | Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	55,708	55,754
Amendments	0	(1,446)
Service cost	1,023	847
Interest cost	2,314	2,120
Curtailment Loss / (Gain)	0	0
Actuarial loss (gain)	(6,756)	(1,122)
Benefits paid	2,801	5,581
Medicare part D subsidy received	0	0
Administrative expense	0	0
Benefit obligation at end of period	63,000	55,708
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	0	0
Actual return on plan assets	0	0
Employer contribution	2,801	5,581
Benefits paid	(2,801)	(5,581)
Administrative expense	0	0
Fair value of plan assets at end of period	0	0
Funded status at end of period	(63,000)	(55,708)
Qualified Plan | Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	677,643	700,481
Amendments	0	0
Service cost	22,656	22,757
Interest cost	28,913	27,303
Curtailment Loss / (Gain)	0	0
Actuarial loss (gain)	(84,272)	29,067
Benefits paid	36,740	42,662
Medicare part D subsidy received	0	0
Administrative expense	(2,439)	(1,169)
Benefit obligation at end of period	774,305	677,643
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	640,242	704,360
Actual return on plan assets	133,939	(38,379)
Employer contribution	18,000	18,000
Benefits paid	(36,740)	(42,662)
Administrative expense	(2,399)	(1,077)
Fair value of plan assets at end of period	753,042	640,242
Funded status at end of period	$ (21,263)	$ (37,401)