Segment Information - Schedule of Selected Information for Each Business Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 331,378	$ 301,853	$ 293,461
Operating income
Operating income	51,853	56,588	42,530
Interest expense	18,410	26,618	38,715
Debt retirement costs	0	1,896	0
Loss on extinguishment of debt	(2,442)	0	(15,528)
Other (income) expense	8,638	220	(65)
Income (loss) before income taxes	39,639	28,294	(11,778)
Depreciation and amortization	18,189	17,330	18,904
Long-lived assets	92,999	94,187
DEFINITY
Segment Reporting Information [Line Items]
Revenues	157,268	131,612	111,859
TechneLite
Segment Reporting Information [Line Items]
Revenues	104,644	99,217	72,562
Xenon
Segment Reporting Information [Line Items]
Revenues	31,377	29,086	48,898
Other
Segment Reporting Information [Line Items]
Revenues	38,089	41,938	60,142
U.S.
Segment Reporting Information [Line Items]
Revenues	290,002	257,420	235,824
Canada
Segment Reporting Information [Line Items]
Revenues	18,770	18,918	28,340
All other
Segment Reporting Information [Line Items]
Revenues	22,606	25,515	29,297
U.S.
Segment Reporting Information [Line Items]
Revenues	290,002	257,420	235,824
Operating income
Operating income	49,239	46,909	42,008
Depreciation and amortization	17,672	15,995	17,054
Long-lived assets	91,537	92,650
International
Segment Reporting Information [Line Items]
Revenues	41,376	44,433	57,637
Operating income
Operating income	2,614	9,679	522
Depreciation and amortization	517	1,335	$ 1,850
Long-lived assets	$ 1,462	$ 1,537