Shareholders' Equity | Company's Common Share Repurchase and Issuance Activity (Details) - $ / shares	3 Months Ended
	Apr. 30, 2018	Jan. 31, 2018
Increase (Decrease) in Treasury Stock
Treasury stock balance at January 31, 2018	21,083,972
Shares repurchased	0
Treasury stock balance at April 30, 2018	20,927,954
Treasury Stock Held, Average Cost Per Share	$ 44.59	$ 44.59
Equity Incentive Plans
Increase (Decrease) in Treasury Stock
Shares of treasury stock reissued for equity incentive plans	156,018