Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,294	$ 2,163
Interest cost	3,858	3,929
Expected return on plan assets	(5,036)	(4,889)
Recognized net actuarial loss	1,535	1,361
Amortization of prior service cost	(107)	(172)
Net periodic benefit cost (benefit)	2,544	2,392
Estimated future employer contributions in current fiscal year	9,200
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	187	175
Interest cost	167	172
Expected return on plan assets	(1,312)	(1,222)
Recognized net actuarial loss	0	0
Amortization of prior service cost	(1,662)	(1,662)
Net periodic benefit cost (benefit)	$ (2,620)	$ (2,537)