Quarterly Information (Unaudited) (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
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 161,844,000	$ 149,640,000	$ 139,398,000	$ 132,836,000	$ 148,921,000	$ 135,946,000	$ 132,139,000	$ 123,156,000	$ 583,717,713	$ 540,161,682	$ 491,445,103
Provision for loan losses	20,911,000	37,395,000	32,402,000	23,615,000	16,700,000	36,109,000	30,057,000	22,839,000	114,322,525	105,705,536	95,908,363
General and administrative expenses	75,595,000	74,798,000	66,158,000	69,159,000	68,474,000	66,234,000	61,464,000	64,513,000	285,710,394	260,684,157	237,514,934
Interest expense	4,998,000	4,404,000	4,066,000	3,926,000	3,230,000	3,338,000	3,947,000	3,384,000	17,393,963	13,898,648	14,772,694
Income tax expense	22,440,000	12,369,000	13,871,000	13,521,000	22,891,000	10,683,000	13,367,000	12,238,000	62,201,083	59,178,898	51,999,932
Net income	$ 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
Earnings per share:
Basic (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
Diluted (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