Pension Plan and Postretirement Benefits Other Than Pension (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Components of net periodic benefit cost:
Percentage of net periodic pension and other postretirement benefit costs included in Compensation and related costs				60.00%
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Final number of years of employee's compensation to determine the benefits payable				10 years
Employer contributions		$ 20,000
Components of net periodic benefit cost:
Service cost	$ 3,106		$ 2,994	$ 6,099	$ 6,041
Interest cost	2,277		2,056	4,716	4,210
Expected return on plan assets	(3,427)		(3,565)	(6,963)	(7,256)
Actuarial (gain) loss amortization	1,469		1,209	3,107	2,585
Prior service cost amortization	93		115	187	230
Transition obligation amortization	1		1	2	2
Total	3,519		2,810	$ 7,148	5,812
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits				65 years
Components of net periodic benefit cost:
Service cost	185		228	$ 370	456
Interest cost	92		99	184	198
Actuarial (gain) loss amortization	(38)			(77)
Prior service cost amortization	1		5	2	10
Total	$ 240		$ 332	$ 479	$ 664