Retirement Plans - Projected benefit obligation and plan assets (Details) - Defined benefit plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Change in projected benefit obligation:
Projected benefit obligation, beginning of year	$ 122,165		$ 93,124
Projected benefit obligation assumed due to Catlin Acquisition	0		28,414
Service cost	1,701	[1]	1,494	[1]	$ 1,279
Interest cost	5,126		4,382		3,747
Actuarial (gain) loss	7,948		2,821
Benefits and expenses paid	(5,977)		(3,747)
Foreign currency (gains) losses	(10,248)		(4,047)
Settlements	(677)		(276)
Projected benefit obligation, end of year	120,038		122,165		93,124
Change in plan assets:
Fair value of plan assets - beginning of year	79,402		49,281
Fair value of plan assets acquired due to Catlin Acquisition	0		33,131
Actual return on plan assets	8,499		(119)
Employer contributions	2,077		1,775
Benefits and expenses paid	(4,577)		(3,305)
Foreign currency (gains) losses	(8,149)		(1,361)
Fair value of plan assets - end of year	77,252		79,402		$ 49,281
Funded status - end of year	(42,786)		(42,763)
Accrued pension liability	$ 42,786		$ 42,763

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