Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,996	$ 2,562
Interest cost	3,955	3,782
Expected return on plan assets	(4,908)	(5,570)
Amortization of net actuarial losses/(gains)	2,167	1,785
Amortization of prior service cost/(credit)	(70)	(87)
Net periodic benefit cost (benefit)	3,140	2,472
Estimated future employer contributions in current fiscal year	10,700
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	312	270
Interest cost	291	214
Expected return on plan assets	(1,445)	(1,588)
Amortization of net actuarial losses/(gains)	0	(46)
Amortization of prior service cost/(credit)	(8)	(1,026)
Net periodic benefit cost (benefit)	$ (850)	$ (2,176)