Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,389	$ 2,283	$ 7,167	$ 6,848
Interest cost	3,869	3,976	11,606	11,929
Expected return on plan assets	(5,025)	(4,877)	(15,074)	(14,631)
Recognized net actuarial loss	1,543	1,464	4,627	4,392
Amortization of prior service cost	(107)	(172)	(320)	(515)
Net periodic benefit cost (benefit)	2,669	2,674	8,006	8,023
Employer contributions in current fiscal year			9,300
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	203	188	610	564
Interest cost	176	176	529	528
Expected return on plan assets	(1,312)	(1,222)	(3,936)	(3,665)
Recognized net actuarial loss	0	0	0	0
Amortization of prior service cost	(1,662)	(1,662)	(4,987)	(4,987)
Net periodic benefit cost (benefit)	$ (2,595)	$ (2,520)	$ (7,784)	$ (7,560)