Summary of Significant Accounting Policies (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Weighted-average shares outstanding for basic and diluted net income per share
Net income	$ 15,073	$ 21,105	$ 20,496	$ 18,983	$ 18,553	$ 17,531	$ 13,430	$ 12,092	$ 75,657	$ 61,606	$ 48,379
Basic net income per share:
Weighted-average shares outstanding - Basic	59,048	58,847	58,511	58,362	58,010	57,956	57,870	57,792	58,729	57,894	57,270
Basic net income per common share	$ 0.26	$ 0.36	$ 0.35	$ 0.33	$ 0.32	$ 0.30	$ 0.23	$ 0.21	$ 1.29	$ 1.06	$ 0.84
Diluted net income per share:
Weighted-average shares outstanding - Basic	59,048	58,847	58,511	58,362	58,010	57,956	57,870	57,792	58,729	57,894	57,270
Effect of potentially dilutive securities									320	342	322
Weighted-average shares outstanding - Diluted	59,232	59,128	58,902	58,800	58,404	58,280	58,156	58,114	59,049	58,236	57,592
Diluted net income per common share	$ 0.25	$ 0.36	$ 0.35	$ 0.32	$ 0.32	$ 0.30	$ 0.23	$ 0.21	$ 1.28	$ 1.06	$ 0.84