Income Taxes (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of variations between the effective and statutory U.S. federal income tax rates
Federal income tax provision at statutory tax rate	$ 40,503	$ 36,713	$ 35,518
State tax provision on income less applicable federal tax benefit	1,470	2,413	1,807
Foreign income taxed at different rates	(1,172)	(900)	(1,144)
Effect of equity in foreign joint venture			954
Repatriation of foreign earnings	24		543
Domestic production activities deduction	(1,446)	(1,339)	(1,446)
Nontaxable foreign interest income	(2,690)	(2,719)	(81)
U.S. tax credits		(1,482)	(969)
Non-deductible expenses and other items, net	(654)	(394)	706
Total income tax provision	$ 36,035	$ 32,292	$ 35,888
Percentage of Federal income tax provision at statutory tax rate	35.00%	35.00%	35.00%
Percentage of State tax provision on income less applicable federal tax benefit	1.30%	2.30%	1.80%
Percentage of Foreign income taxed at different rates	(1.00%)	(0.90%)	(1.10%)
Percentage of Effect of equity in foreign joint venture			0.90%
Percentage of Repatriation of foreign earnings			0.50%
Percentage of Domestic production activities deduction	(1.20%)	(1.30%)	(1.40%)
Percentage of Nontaxable foreign interest income	(2.30%)	(2.60%)	(0.10%)
Percentage of U.S. tax credits		(1.40%)	(1.00%)
Percentage of Non-deductible expenses and other items, net	(0.70%)	(0.30%)	0.80%
Percentage of Total income tax provision	31.10%	30.80%	35.40%