CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss)	Noncontrolling interest in subsidiary
Beginning Balance at Dec. 31, 2017	$ 353,078	$ 6,089	$ 300,866	$ 220,104	$ (191,737)	$ 17,756
Change in accounting principle - ASU 2016-01 at Dec. 31, 2017				46,069	(46,069)
Balance at January 1, 2018, as adjusted at Dec. 31, 2017	353,078	6,089	300,866	266,173	(237,806)	17,756
Net income (loss)	14,736			14,249		487
Other comprehensive income (loss), net of tax	3,501				3,501
Dividends paid to noncontrolling interest	(83)					(83)
Ending Balance at Mar. 31, 2018	371,232	6,089	300,866	280,422	(234,305)	18,160
Beginning Balance at Dec. 31, 2017	353,078	6,089	300,866	220,104	(191,737)	17,756
Change in accounting principle - ASU 2016-01 at Dec. 31, 2017				46,069	(46,069)
Balance at January 1, 2018, as adjusted at Dec. 31, 2017	353,078	6,089	300,866	266,173	(237,806)	17,756
Net income (loss)	(42,098)
Other comprehensive income (loss), net of tax	299
Ending Balance at Sep. 30, 2018	311,320	6,090	301,139	222,421	(237,507)	19,177
Beginning Balance at Mar. 31, 2018	371,232	6,089	300,866	280,422	(234,305)	18,160
Net income (loss)	(41,962)			(42,615)		653
Other comprehensive income (loss), net of tax	(5,199)				(5,199)
Issuance of NL common stock	120	1	119
Dividends paid to noncontrolling interest	(84)					(84)
Other, net	172		154			18
Ending Balance at Jun. 30, 2018	324,279	6,090	301,139	237,807	(239,504)	18,747
Net income (loss)	(14,872)			(15,386)		514
Other comprehensive income (loss), net of tax	1,997				1,997
Dividends paid to noncontrolling interest	(84)					(84)
Ending Balance at Sep. 30, 2018	311,320	6,090	301,139	222,421	(237,507)	19,177
Beginning Balance at Dec. 31, 2018	303,558	6,090	301,139	225,156	(248,270)	19,443
Net income (loss)	15,726			15,204		522
Other comprehensive income (loss), net of tax	733				733
Dividends paid to noncontrolling interest	(117)					(117)
Ending Balance at Mar. 31, 2019	319,900	6,090	301,139	240,360	(247,537)	19,848
Beginning Balance at Dec. 31, 2018	303,558	6,090	301,139	225,156	(248,270)	19,443
Net income (loss)	21,288
Other comprehensive income (loss), net of tax	(542)
Ending Balance at Sep. 30, 2019	323,461	6,094	299,102	244,700	(248,812)	22,377
Beginning Balance at Mar. 31, 2019	319,900	6,090	301,139	240,360	(247,537)	19,848
Net income (loss)	6,560			5,878		682
Other comprehensive income (loss), net of tax	2,472				2,472
Issuance of NL common stock	100	4	96
Dividends paid to noncontrolling interest	(118)					(118)
Equity transactions with noncontrollinginterest, net and other	(288)		(1,831)			1,543
Ending Balance at Jun. 30, 2019	328,626	6,094	299,404	246,238	(245,065)	21,955
Net income (loss)	(998)			(1,538)		540
Other comprehensive income (loss), net of tax	(3,747)				(3,747)
Dividends paid to noncontrolling interest	(118)					(118)
Other, net	(302)		(302)
Ending Balance at Sep. 30, 2019	$ 323,461	$ 6,094	$ 299,102	$ 244,700	$ (248,812)	$ 22,377