Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,402	$ 2,680	$ 4,565	$ 5,128
Interest cost	4,024	4,016	7,953	7,924
Expected return on plan assets	(4,865)	(5,259)	(9,754)	(10,554)
Recognized net actuarial loss (gain)	1,567	1,533	2,928	2,742
Amortization of prior service cost	(171)	(211)	(343)	(422)
Net periodic benefit cost (benefit)	2,957	2,759	5,349	4,818
Estimated future employer contributions in current fiscal year			11,400
Company contributions			2,600
Other Postretirement Benefits
Components of Net Periodic Benefit Cost [Abstract]
Service cost	201	214	376	416
Interest cost	180	171	352	349
Expected return on plan assets	(1,221)	(1,247)	(2,443)	(2,495)
Recognized net actuarial loss (gain)	0	(53)	0	(88)
Amortization of prior service cost	(1,663)	(1,663)	(3,325)	(3,325)
Net periodic benefit cost (benefit)	$ (2,503)	$ (2,578)	$ (5,040)	$ (5,143)