Defined Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014 item	Jun. 30, 2013	Jun. 30, 2014 item	Jun. 30, 2013
Defined benefit plans disclosure
Number of defined benefit pension plans	3		3
United Kingdom Defined Benefit Pension Plans
Defined benefit plans disclosure
Number of defined benefit pension plans	2		2
Sponsored defined benefit pension plans, number of subsidiaries	2		2
Components of Net Periodic Benefit Cost:
Service cost	$ 580	$ 458	$ 1,150	$ 922
Interest cost	2,373	1,951	4,708	3,931
Expected return on plan assets	(3,369)	(2,484)	(6,684)	(5,005)
Amortization of net actuarial loss	373	153	740	309
Net periodic benefit (income) cost	(43)	78	(86)	157
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	4.60%	4.60%	4.60%	4.60%
Expected rate of return on assets (as a percent)			6.15%	5.30%
Salary increases (as a percent)	4.00%	3.60%	4.00%	3.60%
German Defined Benefit Pension Plan
Defined benefit plans disclosure
Number of defined benefit pension plans	1		1
Sponsored defined benefit pension plans, number of subsidiaries	1		1
Components of Net Periodic Benefit Cost:
Service cost	203	192	406	386
Interest cost	174	151	348	304
Amortization of net actuarial loss	62	56	124	113
Net periodic benefit (income) cost	439	399	878	803
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	3.50%	3.50%	3.50%	3.50%
Salary increases (as a percent)	2.00%	2.00%	2.00%	2.00%
Supplemental Executive Retirement Plan
Defined benefit plans disclosure
Number of defined benefit pension plans	1		1
Components of Net Periodic Benefit Cost:
Service cost	402	377	804	754
Interest cost	248	181	496	362
Amortization of prior service credit	(30)	(30)	(60)	(60)
Amortization of net actuarial loss	148	142	296	284
Net periodic benefit cost	768	670	1,536	1,340
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	3.90%	3.20%	3.90%	3.20%
Salary increases (as a percent)	3.25%	3.25%	3.25%	3.25%
Post-Employment Retiree Health and Welfare Plan
Defined benefit plans disclosure
Number of defined benefit pension plans	1		1
Components of Net Periodic Benefit Cost:
Service cost	11	18	22	36
Interest cost	63	59	125	119
Net periodic benefit cost	$ 74	$ 77	$ 147	$ 155
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	4.40%	3.60%	4.40%	3.60%
Health care cost trend rate (as a percent)	7.00%	7.50%	7.00%	7.50%
Health care cost ultimate trend rate (as a percent)	5.00%		5.00%