Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Feb. 28, 2014	Feb. 28, 2013	Feb. 28, 2014	Feb. 28, 2013
Basic Earnings per Share
Net income	$ 84,602	$ 74,705	$ 247,218	$ 229,465
Less dividends to:
Dividends	0	0	93,314	79,744
Undistributed net income	84,602	74,705	153,904	149,721
Less: net income allocated to participating unvested securities	685	517	1,237	1,048
Net income available to common shareholders	83,917	74,188	152,667	148,673
Basic weighted average common shares outstanding (shares)	119,913	123,120	120,658	124,483
Basic earnings per common share:
Total common shares, basic (in dollars per share)	$ 0.70	$ 0.60	$ 2.04	$ 1.84
Diluted Earnings per Share
Net income	84,602	74,705	247,218	229,465
Less dividends to:
Dividends	0	0	93,314	79,744
Undistributed net income	84,602	74,705	153,904	149,721
Less: net income allocated to participating unvested securities	685	517	1,237	1,048
Net income available to common shareholders	83,917	74,188	152,667	148,673
Basic weighted average common shares outstanding (shares)	119,913	123,120	120,658	124,483
Effect of dilutive securities - employee stock options (shares)	1,367	637	1,156	418
Diluted weighted average common shares outstanding (shares)	121,280	123,757	121,814	124,901
Diluted earnings per share:
Total common shares, diluted (in dollars per share)	$ 0.69	$ 0.60	$ 2.02	$ 1.83
Common Stock
Less dividends to:
Dividends	0	0	92,244	78,866
Basic earnings per common share:
Earnings per share, basic - distributed earnings (in dollars per share)	$ 0.00	$ 0.00	$ 0.77	$ 0.64
Earnings per share, basic - undistributed earnings (in dollars per share)	$ 0.70	$ 0.60	$ 1.27	$ 1.20
Total common shares, basic (in dollars per share)	$ 0.70	$ 0.60	$ 2.04	$ 1.84
Less dividends to:
Dividends	0	0	92,244	78,866
Diluted earnings per share:
Earnings per share, diluted - distributed earnings (in dollars per share)	$ 0.00	$ 0.00	$ 0.77	$ 0.64
Earnings per share, diluted - undistributed earnings (in dollars per share)	$ 0.69	$ 0.60	$ 1.25	$ 1.19
Total common shares, diluted (in dollars per share)	$ 0.69	$ 0.60	$ 2.02	$ 1.83
Nonvested Stock
Less dividends to:
Dividends	0	0	1,070	878
Basic earnings per common share:
Earnings per share, basic - distributed earnings (in dollars per share)	$ 0.00	$ 0.00	$ 0.77	$ 0.64
Earnings per share, basic - undistributed earnings (in dollars per share)	$ 0.70	$ 0.60	$ 1.27	$ 1.20
Total common shares, basic (in dollars per share)	$ 0.70	$ 0.60	$ 2.04	$ 1.84
Less dividends to:
Dividends	$ 0	$ 0	$ 1,070	$ 878
Diluted earnings per share:
Earnings per share, diluted - distributed earnings (in dollars per share)	$ 0.00	$ 0.00	$ 0.77	$ 0.64
Earnings per share, diluted - undistributed earnings (in dollars per share)	$ 0.69	$ 0.60	$ 1.25	$ 1.19
Total common shares, diluted (in dollars per share)	$ 0.69	$ 0.60	$ 2.02	$ 1.83