Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,484	$ 2,402	$ 4,778	$ 4,565
Interest cost	3,879	4,024	7,737	7,953
Expected return on plan assets	(5,013)	(4,865)	(10,049)	(9,754)
Recognized net actuarial loss	1,549	1,567	3,084	2,928
Amortization of prior service cost	(106)	(171)	(213)	(343)
Net periodic benefit cost (benefit)	2,793	2,957	5,337	5,349
Estimated future employer contributions in current fiscal year	9,400		9,400
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	220	201	407	376
Interest cost	186	180	353	352
Expected return on plan assets	(1,312)	(1,221)	(2,624)	(2,443)
Recognized net actuarial loss	0	0	0	0
Amortization of prior service cost	(1,663)	(1,663)	(3,325)	(3,325)
Net periodic benefit cost (benefit)	$ (2,569)	$ (2,503)	$ (5,189)	$ (5,040)