Pension Plan and Postretirement Benefits Other Than Pension (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
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	$ 2,994		$ 2,317	$ 6,041	$ 5,042
Interest cost	2,056		2,045	4,210	4,198
Expected return on plan assets	(3,565)		(3,595)	(7,256)	(7,008)
Actuarial (gain) loss amortization	1,209		337	2,585	748
Prior service cost amortization	115		104	230	234
Transition obligation amortization	1		1	2	2
Total	2,810		1,209	$ 5,812	3,216
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	228		180	$ 456	360
Interest cost	99		99	198	198
Actuarial (gain) loss amortization			(4)		(8)
Prior service cost amortization	5		14	10	28
Total	$ 332		$ 289	$ 664	$ 578