Pension Plan and Postretirement Benefits Other Than Pension (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	2,726	$ 2,993
Interest cost	1,654	2,439
Expected return on plan assets	(2,559)	(3,536)
Actuarial (gain) loss amortization	1,265	1,638
Prior service cost amortization	31	94
Transition obligation amortization	1	1
Total	3,118	3,629
Pension Benefits | Compensation and Related Costs
Components of net periodic benefit cost:
Total	$ 1,900
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
Interest cost	$ 15	92
Actuarial (gain) loss amortization	(45)	(39)
Prior service cost amortization	(1)	1
Total	(31)	$ 239
Other Postretirement Benefits | Underwriting and Distribution Expense
Components of net periodic benefit cost:
Total	$ 1,200