Income Tax (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Current
Federal taxes									$ 30,382	$ 36,109	$ 28,979
State taxes									5,019	8,614	6,501
Foreign taxes									190	73	0
Total current taxes									35,591	44,796	35,480
Deferred
Federal taxes									(8,469)	(3,571)	(2,168)
State taxes									(742)	(502)	(502)
Foreign taxes									(190)	(73)	0
Total deferred taxes									(9,401)	(4,146)	(2,670)
Provision for income taxes	$ 1,898	$ 7,649	$ 8,811	$ 7,832	$ 8,521	$ 11,247	$ 11,002	$ 9,880	$ 26,190	$ 40,650	$ 32,810