Retirement Plans - Projected benefit obligation and plan assets (Details) - Defined benefit plan - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in projected benefit obligation:
Projected benefit obligation, beginning of year		$ 120,038	$ 122,165	$ 93,124
Projected benefit obligation assumed due to Catlin Acquisition		0	0	28,414
Service cost	[1]	1,296	1,701	1,494
Interest cost		3,620	5,126	4,382
Actuarial (gain) loss		(958)	7,948	2,821
Benefits and expenses paid		(5,831)	(5,977)	(3,747)
Foreign currency (gains) losses		7,531	(10,248)	(4,047)
Settlements		(1,270)	(677)	(276)
Projected benefit obligation, end of year		124,426	120,038	122,165
Change in plan assets:
Fair value of plan assets - beginning of year		77,252	79,402	49,281
Fair value of plan assets acquired due to Catlin Acquisition		0	0	33,131
Actual return on plan assets		7,900	8,499	(119)
Employer contributions		1,588	2,077	1,775
Benefits and expenses paid		(4,769)	(4,577)	(3,305)
Foreign currency (gains) losses		3,877	(8,149)	(1,361)
Fair value of plan assets - end of year		85,848	77,252	79,402
Funded status - end of year		(38,578)	(42,786)	(42,763)
Accrued pension liability		$ 38,578	$ 42,786	$ 42,763

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