Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,703	$ 2,484	$ 5,265	$ 4,778
Interest cost	3,765	3,879	7,547	7,737
Expected return on plan assets	(5,555)	(5,013)	(11,125)	(10,049)
Recognized net actuarial loss	1,684	1,549	3,469	3,084
Amortization of prior service cost	(88)	(106)	(175)	(213)
Net periodic benefit cost (benefit)	2,509	2,793	4,981	5,337
Estimated future employer contributions in current fiscal year	11,000		11,000
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	310	220	580	407
Interest cost	203	186	417	353
Expected return on plan assets	(1,591)	(1,312)	(3,179)	(2,624)
Recognized net actuarial loss	(79)	0	(125)	0
Amortization of prior service cost	(1,026)	(1,663)	(2,052)	(3,325)
Net periodic benefit cost (benefit)	$ (2,183)	$ (2,569)	$ (4,359)	$ (5,189)