Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Components of Net Periodic Benefit Cost [Abstract]
Estimated future employer contributions in current fiscal year	$ 11,400
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	2,163	$ 2,448
Interest cost	3,929	3,908
Expected return on plan assets	(4,889)	(5,295)
Recognized net actuarial loss (gain)	1,361	1,209
Amortization of prior service cost	(172)	(211)
Net periodic benefit cost (benefit)	2,392	2,059
Other Postretirement Benefits
Components of Net Periodic Benefit Cost [Abstract]
Service cost	175	202
Interest cost	172	178
Expected return on plan assets	(1,222)	(1,248)
Recognized net actuarial loss (gain)	0	(35)
Amortization of prior service cost	(1,662)	(1,662)
Net periodic benefit cost (benefit)	$ (2,537)	$ (2,565)