Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,176	$ 2,703	$ 4,172	$ 5,265
Interest cost	3,898	3,765	7,853	7,547
Expected return on plan assets	(4,825)	(5,555)	(9,733)	(11,125)
Amortization of net actuarial losses/(gains)	2,039	1,684	4,206	3,469
Amortization of prior service cost/(credit)	(70)	(88)	(140)	(175)
Net periodic benefit cost (benefit)	3,218	2,509	6,358	4,981
Estimated future employer contributions in current fiscal year	10,200		10,200
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	360	310	672	580
Interest cost	274	203	565	417
Expected return on plan assets	(1,447)	(1,591)	(2,892)	(3,179)
Amortization of net actuarial losses/(gains)	0	(79)	0	(125)
Amortization of prior service cost/(credit)	(9)	(1,026)	(17)	(2,052)
Net periodic benefit cost (benefit)	$ (822)	$ (2,183)	$ (1,672)	$ (4,359)