Schedule I - Condensed Financial Information of the Parent (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities
Net income (loss)	$ 24,131	$ (442)	$ 3,116	$ 2,802	$ 18,091	$ 3,141	$ (275)	$ 1,027	$ 29,607	$ 21,984	$ (38,118)
Net cash provided by operating activities									93,935	108,002	19,799
Investing Activities
Expenditures for property, plant and equipment									(16,167)	(10,615)	(57,500)
Net cash used for investing activities									(9,817)	(8,291)	(74,934)
Financing Activities
Purchase of treasury shares									(6,044)	(24,010)	(35,075)
Cash dividends paid									(7,262)	(7,296)	(7,755)
Other									(3)	922	(5)
Net cash provided by (used for) financing activities									(55,710)	(108,301)	20,979
Cash and Cash Equivalents
Increase (decrease) for the year									28,149	(8,636)	(34,255)
Balance at the beginning of the year				52,499				61,135	52,499	61,135	95,390
Balance at the end of the year	80,648				52,499				80,648	52,499	61,135
Parent Company
Operating Activities
Net income (loss)									29,607	21,984	(38,118)
Equity in earnings of subsidiaries									35,146	25,231	(33,991)
Parent company only net loss									(5,539)	(3,247)	(4,127)
Net changes related to operating activities									2,684	(11,015)	5,710
Net cash provided by operating activities									(2,855)	(14,262)	1,583
Investing Activities
Expenditures for property, plant and equipment									(25)	(328)	(103)
Net cash used for investing activities									(25)	(328)	(103)
Financing Activities
Cash dividends received from subsidiaries									52,200	15,000	22,300
Notes payable to Bellaire									(600)	0	(1,750)
Capital contributions to subsidiaries									0	0	(19,800)
Purchase of treasury shares									(6,044)	(24,010)	(35,075)
Cash dividends paid									(7,262)	(7,296)	(7,755)
Other									(3)	(13)	(20)
Net cash provided by (used for) financing activities									38,291	(16,319)	(42,100)
Cash and Cash Equivalents
Increase (decrease) for the year									35,411	(30,909)	(40,620)
Balance at the beginning of the year				$ 22,506				$ 53,415	22,506	53,415	94,035
Balance at the end of the year	$ 57,917				$ 22,506				$ 57,917	$ 22,506	$ 53,415