Segment Information - Percentage of Total Sales by Product Group (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Revenues	$ 331,096	$ 296,129	$ 996,320	$ 850,669
Breathing Apparatus
Revenue from External Customer [Line Items]
Revenues	80,424	66,175	230,324	204,813
Fixed Gas & Flame Detection
Revenue from External Customer [Line Items]
Revenues	64,951	61,394	189,482	172,350
Portable Gas Detection
Revenue from External Customer [Line Items]
Revenues	39,405	35,496	122,939	109,016
Industrial Head Protection
Revenue from External Customer [Line Items]
Revenues	36,938	34,598	111,532	101,471
Fall Protection
Revenue from External Customer [Line Items]
Revenues	27,939	24,730	79,696	72,185
Firefighter Helmets & Protective Apparel
Revenue from External Customer [Line Items]
Revenues	35,821	31,967	126,981	60,434
Other
Revenue from External Customer [Line Items]
Revenues	$ 45,618	$ 41,769	$ 135,366	$ 130,400
Sales
Revenue from External Customer [Line Items]
Concentration risk percentage	100.00%	100.00%	100.00%	100.00%
Sales | Breathing Apparatus
Revenue from External Customer [Line Items]
Concentration risk percentage	24.00%	22.00%	23.00%	24.00%
Sales | Fixed Gas & Flame Detection
Revenue from External Customer [Line Items]
Concentration risk percentage	20.00%	21.00%	19.00%	20.00%
Sales | Portable Gas Detection
Revenue from External Customer [Line Items]
Concentration risk percentage	12.00%	12.00%	12.00%	13.00%
Sales | Industrial Head Protection
Revenue from External Customer [Line Items]
Concentration risk percentage	11.00%	12.00%	11.00%	12.00%
Sales | Fall Protection
Revenue from External Customer [Line Items]
Concentration risk percentage	8.00%	8.00%	8.00%	8.00%
Sales | Firefighter Helmets & Protective Apparel
Revenue from External Customer [Line Items]
Concentration risk percentage	11.00%	11.00%	13.00%	7.00%
Sales | Other
Revenue from External Customer [Line Items]
Concentration risk percentage	14.00%	14.00%	14.00%	16.00%
Americas
Revenue from External Customer [Line Items]
Revenues	$ 209,343	$ 186,898	$ 633,812	$ 528,426
Americas | Breathing Apparatus
Revenue from External Customer [Line Items]
Revenues	50,282	40,876	146,293	134,835
Americas | Fixed Gas & Flame Detection
Revenue from External Customer [Line Items]
Revenues	33,803	32,409	99,459	91,048
Americas | Portable Gas Detection
Revenue from External Customer [Line Items]
Revenues	26,755	23,786	82,654	73,469
Americas | Industrial Head Protection
Revenue from External Customer [Line Items]
Revenues	28,909	27,539	87,901	80,503
Americas | Fall Protection
Revenue from External Customer [Line Items]
Revenues	16,465	13,727	45,668	39,217
Americas | Firefighter Helmets & Protective Apparel
Revenue from External Customer [Line Items]
Revenues	30,373	25,345	102,906	37,123
Americas | Other
Revenue from External Customer [Line Items]
Revenues	$ 22,756	$ 23,216	$ 68,931	$ 72,231
Americas | Sales
Revenue from External Customer [Line Items]
Concentration risk percentage	100.00%	100.00%	100.00%	100.00%
Americas | Sales | Breathing Apparatus
Revenue from External Customer [Line Items]
Concentration risk percentage	24.00%	22.00%	23.00%	26.00%
Americas | Sales | Fixed Gas & Flame Detection
Revenue from External Customer [Line Items]
Concentration risk percentage	16.00%	17.00%	16.00%	17.00%
Americas | Sales | Portable Gas Detection
Revenue from External Customer [Line Items]
Concentration risk percentage	13.00%	13.00%	13.00%	14.00%
Americas | Sales | Industrial Head Protection
Revenue from External Customer [Line Items]
Concentration risk percentage	14.00%	15.00%	14.00%	15.00%
Americas | Sales | Fall Protection
Revenue from External Customer [Line Items]
Concentration risk percentage	8.00%	7.00%	7.00%	7.00%
Americas | Sales | Firefighter Helmets & Protective Apparel
Revenue from External Customer [Line Items]
Concentration risk percentage	15.00%	14.00%	16.00%	7.00%
Americas | Sales | Other
Revenue from External Customer [Line Items]
Concentration risk percentage	10.00%	12.00%	11.00%	14.00%
International
Revenue from External Customer [Line Items]
Revenues	$ 121,753	$ 109,231	$ 362,508	$ 322,243
International | Breathing Apparatus
Revenue from External Customer [Line Items]
Revenues	30,142	25,299	84,031	69,978
International | Fixed Gas & Flame Detection
Revenue from External Customer [Line Items]
Revenues	31,148	28,985	90,023	81,302
International | Portable Gas Detection
Revenue from External Customer [Line Items]
Revenues	12,650	11,710	40,285	35,547
International | Industrial Head Protection
Revenue from External Customer [Line Items]
Revenues	8,029	7,059	23,631	20,968
International | Fall Protection
Revenue from External Customer [Line Items]
Revenues	11,474	11,003	34,028	32,968
International | Firefighter Helmets & Protective Apparel
Revenue from External Customer [Line Items]
Revenues	5,448	6,622	24,075	23,311
International | Other
Revenue from External Customer [Line Items]
Revenues	$ 22,862	$ 18,553	$ 66,435	$ 58,169
International | Sales
Revenue from External Customer [Line Items]
Concentration risk percentage	100.00%	100.00%	100.00%	100.00%
International | Sales | Breathing Apparatus
Revenue from External Customer [Line Items]
Concentration risk percentage	25.00%	23.00%	23.00%	22.00%
International | Sales | Fixed Gas & Flame Detection
Revenue from External Customer [Line Items]
Concentration risk percentage	26.00%	27.00%	25.00%	25.00%
International | Sales | Portable Gas Detection
Revenue from External Customer [Line Items]
Concentration risk percentage	10.00%	11.00%	11.00%	11.00%
International | Sales | Industrial Head Protection
Revenue from External Customer [Line Items]
Concentration risk percentage	7.00%	6.00%	7.00%	7.00%
International | Sales | Fall Protection
Revenue from External Customer [Line Items]
Concentration risk percentage	9.00%	10.00%	9.00%	10.00%
International | Sales | Firefighter Helmets & Protective Apparel
Revenue from External Customer [Line Items]
Concentration risk percentage	4.00%	6.00%	7.00%	7.00%
International | Sales | Other
Revenue from External Customer [Line Items]
Concentration risk percentage	19.00%	17.00%	18.00%	18.00%