Pension Plan and Postretirement Benefits Other Than Pension (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
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			$ 10,000
Components of net periodic benefit cost:
Service cost	$ 2,683	$ 3,106	5,409	$ 6,099
Interest cost	1,604	2,277	3,258	4,716
Expected return on plan assets	(2,558)	(3,427)	(5,117)	(6,963)
Actuarial (gain) loss amortization	1,169	1,469	2,434	3,107
Prior service cost (credit) amortization	31	93	62	187
Transition obligation amortization	1	1	2	2
Total	2,930	3,519	6,048	7,148
Pension Benefits | Compensation and Related Costs
Components of net periodic benefit cost:
Total	1,800		$ 3,700
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		370
Interest cost	14	92	$ 29	184
Actuarial (gain) loss amortization	(45)	(38)	(90)	(77)
Prior service cost (credit) amortization	(1)	1	(2)	2
Total	(32)	$ 240	$ (63)	479
Other Postretirement Benefits | Underwriting and Distribution Expense
Components of net periodic benefit cost:
Total	$ 1,100			$ 2,300