Summary of Significant Accounting Policies (Details) (USD $) In Thousands, except Per Share data, 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
Weighted-average shares outstanding for basic and diluted net income per share
Net income	$ (4,090)		$ 15,626		$ 15,691		$ 15,497		$ 15,073		$ 21,105		$ 20,496		$ 18,983		$ 42,724	$ 75,657	$ 61,606
Basic net income per share:
Weighted-average shares outstanding - Basic	59,541		59,400		59,347		59,281		59,048		58,847		58,511		58,362		59,392	58,729	57,894
Basic net income per common share	$ (0.07)	[1]	$ 0.26	[1]	$ 0.26	[1]	$ 0.26	[1]	$ 0.26	[1]	$ 0.36	[1]	$ 0.35	[1]	$ 0.33	[1]	$ 720.00	$ 1,290.00	$ 1,060.00
Diluted net income per share:
Weighted-average shares outstanding - Basic	59,541		59,400		59,347		59,281		59,048		58,847		58,511		58,362		59,392	58,729	57,894
Effect of potentially dilutive securities																	70	320	342
Weighted-average shares outstanding - Diluted	59,541		59,405		59,386		59,388		59,232		59,128		58,902		58,800		59,462	59,049	58,236
Diluted net income per common share	$ (0.07)	[1]	$ 0.26	[1]	$ 0.26	[1]	$ 0.26	[1]	$ 0.25	[1]	$ 0.36	[1]	$ 0.35	[1]	$ 0.32	[1]	$ 720.00	$ 1,280.00	$ 1,060.00

[1] Quarterly EPS may not sum to annual EPS due to rounding