Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,562	$ 2,294
Interest cost	3,782	3,858
Expected return on plan assets	(5,570)	(5,036)
Recognized net actuarial loss	1,785	1,535
Amortization of prior service cost	(87)	(107)
Net periodic benefit cost (benefit)	2,472	2,544
Estimated future employer contributions in current fiscal year	11,000
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	270	187
Interest cost	214	167
Expected return on plan assets	(1,588)	(1,312)
Recognized net actuarial loss	(46)	0
Amortization of prior service cost	(1,026)	(1,662)
Net periodic benefit cost (benefit)	$ (2,176)	$ (2,620)