Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,633	$ 2,389	$ 7,898	$ 7,167
Interest cost	3,774	3,869	11,321	11,606
Expected return on plan assets	(5,563)	(5,025)	(16,688)	(15,074)
Amortization of net actuarial losses/(gains)	1,734	1,543	5,203	4,627
Amortization of prior service cost/(credit)	(88)	(107)	(263)	(320)
Net periodic benefit cost (benefit)	2,490	2,669	7,471	8,006
Estimated future employer contributions in current fiscal year	11,000		11,000
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	290	203	870	610
Interest cost	208	176	625	529
Expected return on plan assets	(1,590)	(1,312)	(4,769)	(3,936)
Amortization of net actuarial losses/(gains)	(62)	0	(187)	0
Amortization of prior service cost/(credit)	(1,026)	(1,662)	(3,078)	(4,987)
Net periodic benefit cost (benefit)	$ (2,180)	$ (2,595)	$ (6,539)	$ (7,784)