Condensed Consolidating Financial Information - Guarantor - Subsidiaries (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Comprehensive Income [Abstract]
Net earnings	$ 3,441	$ 6,384	$ 6,054	$ 2,178	$ 3,218	$ 3,582	$ 723	$ (2,671)	$ 18,057	$ 4,852	$ 7,117
Unrealized gain on short-term investments									55	81	(142)
Changes in pension plan assets and benefit obligations									(13)	(12)	(7)
Total comprehensive income									18,099	4,921	6,968
Parent Company Only [Member]
Statement of Comprehensive Income [Abstract]
Net earnings									18,057	4,852	7,117
Unrealized gain on short-term investments									55	81	(142)
Changes in pension plan assets and benefit obligations									(13)	(12)	(7)
Total comprehensive income									18,099	4,921	6,968
Guarantor Subsidiaries [Member]
Statement of Comprehensive Income [Abstract]
Net earnings									15,714	5,878	3,823
Unrealized gain on short-term investments
Changes in pension plan assets and benefit obligations
Total comprehensive income									15,714	5,878	3,823
Eliminations [Member]
Statement of Comprehensive Income [Abstract]
Net earnings									(15,714)	(5,878)	(3,823)
Unrealized gain on short-term investments
Changes in pension plan assets and benefit obligations
Total comprehensive income									$ (15,714)	$ (5,878)	$ (3,823)