Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,086	$ 2,633	$ 6,258	$ 7,898
Interest cost	3,926	3,774	11,779	11,321
Expected return on plan assets	(4,866)	(5,563)	(14,599)	(16,688)
Amortization of net actuarial losses/(gains)	2,103	1,734	6,309	5,203
Amortization of prior service cost/(credit)	(71)	(88)	(211)	(263)
Net periodic benefit cost (benefit)	3,178	2,490	9,536	7,471
Estimated employer contributions in current fiscal year	10,200		10,200
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	337	290	1,009	870
Interest cost	283	208	848	625
Expected return on plan assets	(1,447)	(1,590)	(4,339)	(4,769)
Amortization of net actuarial losses/(gains)	0	(62)	0	(187)
Amortization of prior service cost/(credit)	(8)	(1,026)	(25)	(3,078)
Net periodic benefit cost (benefit)	$ (835)	$ (2,180)	$ (2,507)	$ (6,539)