Defined Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 subsidiary plan	Jun. 30, 2012	Jun. 30, 2013 subsidiary plan	Jun. 30, 2012
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	$ 458	$ 584	$ 922	$ 1,163
Interest cost	1,951	1,934	3,931	3,850
Expected return on plan assets	(2,484)	(2,581)	(5,005)	(5,138)
Amortization of net actuarial loss	153	294	309	586
Net periodic benefit cost	78	231	157	461
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)	5.30%	5.90%	5.30%	5.90%
Salary increases (as a percent)	3.60%	4.00%	3.60%	4.00%
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	192	184	386	370
Interest cost	151	171	304	344
Amortization of net actuarial loss	56	8	113	16
Net periodic benefit cost	399	363	803	730
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	3.50%	5.40%	3.50%	5.40%
Salary increases (as a percent)	2.00%	2.50%	2.00%	2.50%
Supplemental Executive Retirement Plan
Defined benefit plans disclosure
Number of defined benefit pension plans	1		1
Components of Net Periodic Benefit Cost:
Service cost	377	370	754	739
Interest cost	181	194	362	388
Amortization of prior service credit	(30)	(30)	(60)	(59)
Amortization of net actuarial loss	142	67	284	134
Net periodic benefit cost	670	601	1,340	1,202
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	3.20%	4.30%	3.20%	4.30%
Salary increases (as a percent)	3.25%	3.75%	3.25%	3.75%
Post-Employment Retiree Health and Welfare Plan
Defined benefit plans disclosure
Number of post-employment retiree health and welfare plan	1		1
Components of Net Periodic Benefit Cost:
Service cost	18	24	36	48
Interest cost	59	73	119	146
Amortization of net actuarial loss		10		19
Net periodic benefit cost	$ 77	$ 107	$ 155	$ 213
Assumptions Used to Determine Net Periodic Benefit Cost:
Discount rate (as a percent)	3.60%	4.60%	3.60%	4.60%
Health care cost trend rate (as a percent)	7.50%	8.00%	7.50%	8.00%
Health care cost ultimate trend rate (as a percent)	5.00%		5.00%