Benefit Plans (Details) (USD $)	12 Months Ended																		0 Months Ended		12 Months Ended																			0 Months Ended									12 Months Ended	0 Months Ended		12 Months Ended					0 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2013 Price Range $15.00-$16.99 [Member]	Mar. 31, 2013 Price Range $23.00-23.99 [Member]	Mar. 31, 2013 Price Range $25.00-25.99 [Member]	Mar. 31, 2013 Price Range $26.00-27.99 [Member]	Mar. 31, 2013 Price Range $28.00-28.99 [Member]	Mar. 31, 2013 Price Range $43.00-43.99 [Member]	Mar. 31, 2013 Price Range $48.00-49.99 [Member]	Mar. 31, 2013 Price Range $66.00-67.99 [Member]	Mar. 31, 2013 Price Range $72.00-73.99 [Member]	Mar. 31, 2013 Price Range $74.00-75.99 [Member]	Mar. 31, 2013 Price Range $76.00-77.99 [Member]	Mar. 31, 2013 Price Range $15.00-77.99 [Member]	Mar. 31, 2013 Stock Options Plans [Member]	Mar. 31, 2012 Stock Options Plans [Member]	Mar. 31, 2011 Stock Options Plans [Member]	Nov. 07, 2011 Restricted Stock [Member]	Nov. 08, 2010 Restricted Stock [Member]	Mar. 31, 2013 Restricted Stock [Member]	Mar. 31, 2012 Restricted Stock [Member]	Mar. 31, 2011 Restricted Stock [Member]	Mar. 31, 2013 Restricted Stock [Member] 15% or higher [Member]	Mar. 31, 2012 Restricted Stock [Member] 15% or higher [Member]	Mar. 31, 2013 Restricted Stock [Member] 12% - 14.99% [Member]	Mar. 31, 2012 Restricted Stock [Member] 12% - 14.99% [Member]	Mar. 31, 2013 Restricted Stock [Member] 10% - 11.99% [Member]	Mar. 31, 2012 Restricted Stock [Member] 10% - 11.99% [Member]	Mar. 31, 2013 Restricted Stock [Member] Below 10% [Member]	Mar. 31, 2012 Restricted Stock [Member] Below 10% [Member]	Mar. 31, 2013 Restricted Stock [Member] Vesting Percentage 100% [Member]	Mar. 31, 2012 Restricted Stock [Member] Vesting Percentage 100% [Member]	Mar. 31, 2013 Restricted Stock [Member] Vesting Percentage 67% [Member]	Mar. 31, 2012 Restricted Stock [Member] Vesting Percentage 67% [Member]	Mar. 31, 2013 Restricted Stock [Member] Vesting Percentage 33% [Member]	Mar. 31, 2012 Restricted Stock [Member] Vesting Percentage 33% [Member]	Mar. 31, 2013 Restricted Stock [Member] Vesting Percentage 0% [Member]	Mar. 31, 2012 Restricted Stock [Member] Vesting Percentage 0% [Member]	Nov. 07, 2011 Restricted Stock [Member] Service Based Award [Member] Officers [Member]	Nov. 07, 2011 Restricted Stock [Member] Service Based Award [Member] Executive Officers [Member]	Nov. 08, 2010 Restricted Stock [Member] Service Based Award [Member] Executive Officers [Member]	Apr. 30, 2012 Restricted Stock [Member] Service Based Award [Member] Independent Directors [Member]	Apr. 29, 2011 Restricted Stock [Member] Service Based Award [Member] Independent Directors [Member]	Feb. 05, 2013 Restricted Stock [Member] Performance Based Awards [Member] Directors and Officers [Member]	Feb. 05, 2013 Restricted Stock [Member] Performance Based Awards [Member] Officers [Member]	Nov. 07, 2011 Restricted Stock [Member] Performance Based Awards [Member] Executive Officers [Member]	Nov. 08, 2010 Restricted Stock [Member] Performance Based Awards [Member] Executive Officers [Member]	Mar. 31, 2013 Restricted Stock [Member] EPS Target [Member] Performance Based Awards [Member]	Dec. 07, 2012 Restricted Stock [Member] EPS Target [Member] Performance Based Awards [Member] Directors and Officers [Member]	Dec. 11, 2012 Restricted Stock [Member] EPS Target [Member] Performance Based Awards [Member] Officers [Member]	Mar. 31, 2013 Restricted Stock [Member] EPS Target [Member] $10.29 EPS [Member]	Mar. 31, 2013 Restricted Stock [Member] EPS Target [Member] $9.76 EPS [Member]	Mar. 31, 2013 Restricted Stock [Member] EPS Target [Member] $9.26 EPS [Member]	Mar. 31, 2013 Restricted Stock [Member] EPS Target [Member] Below $9.26 EPS [Member]	Mar. 31, 2013 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] Performance Based Awards [Member]	Dec. 07, 2012 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] Performance Based Awards [Member] Directors and Officers [Member]	Dec. 11, 2012 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] Performance Based Awards [Member] Officers [Member]	Mar. 31, 2013 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] $13.00 [Member]	Mar. 31, 2013 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] $14.50 [Member]	Mar. 31, 2013 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] $16.00 [Member]	Mar. 31, 2013 Restricted Stock [Member] Trailing Four Quarter EPS Target [Member] $18.00 [Member]	Mar. 31, 2013 Equity Classified Awards [Member]	Mar. 31, 2012 Equity Classified Awards [Member]	Mar. 31, 2011 Equity Classified Awards [Member]	Mar. 31, 2013 Maximum [Member] Stock Options Plans [Member]	Mar. 31, 2013 Maximum [Member] Stock Options Plans [Member] Officers [Member]	Mar. 31, 2013 Maximum [Member] Restricted Stock [Member] EPS Target [Member] Below $9.26 EPS [Member]	Mar. 31, 2013 Supplemental Employee Retirement Plans, Defined Benefit [Member]	Mar. 31, 2012 Supplemental Employee Retirement Plans, Defined Benefit [Member]	Mar. 31, 2011 Supplemental Employee Retirement Plans, Defined Benefit [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum contribution from employer	50.00%
Employer matching contribution, percent	6.00%
Amount of cost recognized	$ 1,432,170	$ 1,290,659	$ 1,175,574
Contribution to supplemental retirement plan																																																																					1,022,979	971,779	924,177
Unfunded liability																																																																					7,470,918	6,732,123	6,044,528
Annual salary increase used for estimating unfunded liability																																																																					3.50%	3.50%	3.50%
Discount rate used for estimating unfunded liability																																																																					6.00%	6.00%	6.00%
Retirement age used for estimating unfunded liability																																																																					65 years	65 years	65 years
Stock Option Plans
Shares of authorized common stock reserved for issuance (in shares)																4,100,000
Vesting period	5 years	5 years	5 years																																																															10 years	5 years
Number of shares available for grant (in shares)																397,188
Weighted-average fair value at the grant date	$ 36.06	$ 35.94	$ 23.96
Options Activity [Roll Forward]
Options outstanding, beginning of year (in shares)																1,041,500
Granted (in shares)																559,000
Exercised (in shares)	(332,665)	(324,140)	(447,250)													(332,665)
Forfeited (in shares)																(17,950)
Expired (in shares)																(300)
Options outstanding, end of period (in shares)																1,249,585	1,041,500
Options exercisable, end of period (in shares)																205,305
Weighted Average Exercise Price [Roll Forward]
Options outstanding, beginning of year (in dollars per share)																$ 37.95
Granted (in dollars per share)																$ 71.43
Exercised (in dollars per share)																$ 29.89
Forfeited (in dollars per share)																$ 49.64
Expired (in dollars per share)																$ 43.04
Options outstanding, end of period (in dollars per share)																$ 54.90	$ 37.95
Options exercisable, end of period (in dollars per share)																$ 33.00
Stock Option Activity Additional Disclosures
Weighted-average remaining contractual term, Options outstanding, end of period																7 years 11 months 23 days
Weighted-average remaining contractual terms, Options exercisable, end of period																4 years 6 months
Aggregate intrinsic value, Options outstanding, end of period																38,696,623
Aggregate intrinsic value, Options exercisable, end of period																10,855,260
Intrinsic value of options exercised																14,049,751	12,049,546	11,151,259
Compensation Cost Not yet Recognized
Total unrecognized stock-based compensation expense related to non-vested stock options																24,300,000
Weighted average period for recognition																4 years 2 months 9 days					3 years 5 months 23 days
Stock Options Outstanding
Options Outstanding (in shares)				92,550	5,500	45,400	125,550	38,250	173,015	13,400	409,920	10,000	333,000	3,000	1,249,585
Weighted Average Remaining Contractual Life				4 years 7 months 21 days	1 year 6 months 29 days	2 years 7 months 17 days	6 years 7 months 10 days	3 years 3 months 18 days	7 years 7 months 10 days	3 years 6 months 29 days	9 years 1 month 17 days	9 years 8 months 12 days	9 years 8 months 9 days	9 years 10 months 10 days	7 years 11 months 23 days
Weighted Average Exercise Price (in dollars per share)				$ 16.57	$ 23.53	$ 25.11	$ 26.73	$ 28.25	$ 43.04	$ 49.00	$ 67.40	$ 73.41	$ 74.08	$ 77.31	$ 54.90
Options Exercisable (in shares)				39,350	5,500	45,400	18,550	38,250	16,295	13,400	28,560	0	0	0	205,305
Weighted Average Exercise Price (in dollars per share)				$ 16.18	$ 23.53	$ 25.11	$ 26.73	$ 28.25	$ 43.04	$ 49.00	$ 67.70	$ 0.00	$ 0.00	$ 0.00	$ 33.00
Restricted Stock
Grants in period (in shares)																																								24,200	15,077	29,080	10,000	10,000		85,000	11,139	15,871	70,800	69,600	1,200					358,700	350,900	7,800
Grant date fair value (in dollars per share)																					$ 74.45																			$ 67.70	$ 67.70	$ 43.04	$ 66.51	$ 67.95	$ 77.31		$ 67.70	$ 43.04		$ 74.08	$ 73.41						$ 74.08	$ 73.41
Award vesting date																																															Apr. 30, 2014	Apr. 30, 2013	Apr. 30, 2015
Ratio of restricted stock vesting immediately (in hundredths)																			33.333%	33.333%
Ratio of restricted stock vesting after year one (in hundredths)																			33.333%	33.333%
Ratio of restricted stock vesting after year two (in hundredths)																			33.333%	33.333%
Schedule of vesting of restricted shares on basis of compounded annual EPS growth [Abstract]
EPS Target (in dollars per share)																																																				10.29	9.76	9.26					13.00	14.50	16.00	18.00						9.26
Vesting Percentage (in hundredths)																								100.00%	100.00%	67.00%	67.00%	33.00%	33.00%	0.00%	0.00%																					100.00%	67.00%	33.00%	0.00%				25.00%	25.00%	25.00%	25.00%
Compounded Annual EPS Growth, Minimum (in hundredths)																																15.00%	15.00%	12.00%	12.00%	10.00%	10.00%
Compounded Annual EPS Growth, Maximum (in hundredths)																																		14.99%	14.99%	11.99%	11.99%	10.00%	10.00%
Compensation related to stock option and restricted stock plans	11,165,327	6,617,827	5,340,707													7,322,653	4,867,231	3,855,348			4,818,956	2,865,857	2,112,721																																								12,141,609	7,733,088	5,968,069
Unrecognized compensation cost related to unvested restricted stock awards granted																					$ 36,300,000
Weighted average period for recognition																4 years 2 months 9 days					3 years 5 months 23 days
Summary of the status and changes in restricted stock [Roll Forward]
Outstanding at beginning of period (in shares)																					93,999
Awards granted (in shares)	524,500	60,416	54,951																		524,500
Vested during the period (in shares)																					(41,285)
Cancelled during the period (in shares)																					(14,758)
Outstanding at end of period (in shares)																					562,456	93,999
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding at beginning of period (in dollars per share)																					$ 50.90
Grant date fair value (in dollars per share)																					$ 74.45																			$ 67.70	$ 67.70	$ 43.04	$ 66.51	$ 67.95	$ 77.31		$ 67.70	$ 43.04		$ 74.08	$ 73.41						$ 74.08	$ 73.41
Vested during the period, net of cancellations (in dollars per share)																					$ 46.30
Cancelled during the period (in dollars per share)																					$ 41.51
Outstanding at end of period (in dollars per share)																					$ 73.32	$ 50.90