Retirement Plans - Net benefit cost (Details) - Defined benefit plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,701	[1]	$ 1,494	[1]	$ 1,279
Interest cost	5,126		4,382		3,747
Expected return on plan assets	(4,357)		(3,903)		(2,859)
Amortization of net actuarial loss	1,705		1,617		812
Net benefit cost	$ 4,175		$ 3,590		$ 2,979

[1] Service costs include cost of living adjustments on curtailed plans.