Benefit Plans (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost		$ 1,876	$ 2,176	$ 3,697	$ 4,172
Interest cost		3,336	3,898	6,750	7,853
Expected return on plan assets		(5,473)	(4,825)	(11,053)	(9,733)
Amortization of net actuarial losses/(gains)		1,526	2,039	3,160	4,206
Amortization of prior service cost/(credit)		(62)	(70)	(124)	(140)
Net periodic benefit cost (benefit)		1,203	3,218	2,430	6,358
Estimated employer contributions in remainder of fiscal year		6,000		6,000
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost		322	360	631	672
Interest cost		202	274	416	565
Expected return on plan assets		(1,852)	(1,447)	(3,704)	(2,892)
Amortization of net actuarial losses/(gains)		(201)	0	(391)	0
Amortization of prior service cost/(credit)		12	(9)	25	(17)
Net periodic benefit cost (benefit)		$ (1,517)	$ (822)	$ (3,023)	$ (1,672)
Subsequent Event | Pension Plan
Components of Net Periodic Benefit Cost [Abstract]
Company contribution to pension plan	$ 6,500