Earnings Per Share (Details) (USD $)	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	Feb. 09, 2012
Income (Numerator)
Income available to common shareholders	$ 37,900,000	$ 20,674,000	$ 22,901,000	$ 22,615,000	$ 37,626,000	$ 19,582,000	$ 23,304,000	$ 20,182,000	$ 104,089,748	$ 100,694,443	$ 91,249,180
Shares (Denominator)
Income available to common shareholders (in shares)									12,940,007	14,906,662	15,833,983
Effect of Dilutive Securities Options and restricted stock (in shares)									274,264	373,489	376,250
Convertible notes payable (in shares)									0	8,960	0
Income available to common shareholders plus assumed exercises of stock options (in shares)									13,214,271	15,289,111	16,210,233
Per Share Amount
Income available to common shareholders (in dollars per share)	$ 3.08	$ 1.61	$ 1.76	$ 1.66	$ 2.60	$ 1.33	$ 1.56	$ 1.30	$ 8.04	$ 6.75	$ 5.76
Income available to common shareholders plus assumed exercises of stock options (in dollars per share)	$ 3.01	$ 1.58	$ 1.72	$ 1.63	$ 2.54	$ 1.30	$ 1.52	$ 1.27	$ 7.88	$ 6.59	$ 5.63
Antidilutive options excluded from computation of diluted earnings per share (in shares)									403,123	86,578	291,755
Strike price of warrants (in dollars per share)												$ 73.97