Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share, Basic and Diluted, by Common Class, Including Two Class Method [Line Items]
Earnings for basic computations									$ 481,085	$ 416,664	$ 299,824
Weighted-average common shares outstanding for basic computations (in shares)									140,059,494	141,910,799	145,964,574
Earnings for diluted computations									$ 481,092	$ 416,675	$ 299,837
Dilutive stock options and restricted stock (in shares)									1,178,641	1,245,377	1,785,448
Weighted-average common shares outstanding for diluted computations (in shares)									141,238,135	143,156,176	147,750,022
Earnings per common share
Basic (in dollars per share)	$ 0.99	$ 0.79	$ 0.81	$ 0.83	$ 0.64	$ 0.92	$ 0.65	$ 0.72	$ 3.43	$ 2.94	$ 2.05
Diluted (in dollars per share)	$ 0.98	$ 0.79	$ 0.80	$ 0.83	$ 0.63	$ 0.92	$ 0.64	$ 0.72	$ 3.41	$ 2.91	$ 2.03
Cash dividends paid									$ 146,602	$ 114,234	$ 103,411
Restricted Stock
Earnings per common share
Unvested shares participating in the payment of the Company's dividends declared									400,000	600,000	800,000
Cash dividends paid									$ 700	$ 600	$ 800
Undistributed earnings (loss) allocated to participating securities, basic									900	1,200	1,100
Undistributed earnings (loss) allocated to participating securities, diluted									$ 900	$ 1,200	$ 1,100
Stock Options
Earnings per common share
Antidilutive options excluded from the computation of EPS (in shares)									200,000	200,000	300,000