Schedule I - Condensed Financial Information of Registrant - Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current assets:
Cash and cash equivalents	$ 98,020,000	$ 124,929,000	$ 66,946,000	$ 61,739,000
Accounts receivable, net	82,801,000	87,351,000
Supplies, prepaid items and other	35,698,000	42,009,000
Total current assets	284,747,000	328,106,000
Property, plant and equipment, net	281,871,000	164,547,000
Investments in and due from subsidiaries	1,809,000	2,910,000
Other assets, net	9,994,000	9,356,000
Total assets	(576,612,000)	(502,009,000)	(387,981,000)
Current liabilities:
Accounts payable	68,333,000	57,891,000
Short-term financing	9,254,000	5,646,000
Accrued and other liabilities	34,698,000	28,677,000
Total current liabilities	117,083,000	97,149,000
Noncurrent accrued and other liabilities	16,369,000	15,239,000
Stockholders' equity:
Common stock	2,673,000	2,664,000
Capital in excess of par value	165,006,000	162,092,000
Retained earnings	212,192,000	153,888,000
Less treasury stock	(28,374,000)	(28,374,000)
Total stockholders' equity	354,497,000	293,270,000	179,370,000	150,607,000
Total Liabilities and Stockholders' equity	(576,612,000)	(502,009,000)
Parent company [Member]
Current assets:
Cash and cash equivalents	22,210,000	35,302,000	7,491,000	23,071,000
Accounts receivable, net	3,000	15,000
Supplies, prepaid items and other	9,579,000	5,833,000
Due from subsidiaries	4,392,000	28,785,000
Note receivable from a subsidiary		10,000,000
Total current assets	36,184,000	79,935,000
Property, plant and equipment, net	202,000	229,000
Investments in and due from subsidiaries	342,897,000	290,303,000
Other assets, net	2,476,000	2,253,000
Total assets	381,759,000	372,720,000
Current liabilities:
Accounts payable	605,000	291,000
Short-term financing	9,254,000	5,646,000
Accrued and other liabilities	1,139,000	1,036,000
Redeemable, noncumulative, convertible preferred stock		44,000
Total current liabilities	10,998,000	7,017,000
Due to subsidiaries		56,243,000
Noncurrent accrued and other liabilities	5,423,000	5,349,000
Stockholders' equity:
Preferred stocks	3,000,000	3,000,000
Common stock	2,673,000	2,664,000
Capital in excess of par value	165,006,000	162,092,000
Retained earnings	212,192,000	153,888,000
Stockholders Equity Including Treasury Stock	382,871,000	321,644,000
Less treasury stock	17,533,000	17,533,000
Total stockholders' equity	365,338,000	304,111,000
Total Liabilities and Stockholders' equity	$ 381,759,000	$ 372,720,000