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Dividends (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Dec. 05, 2018
Nov. 07, 2018
Sep. 04, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]              
Dividends paid per common share (in usd per share)     $ 0.364        
Dividends declared per common share (in usd per share)       $ 0.364 $ 0.362 $ 0.7275 $ 0.7235
Dividends, common stock     $ 85.9        
Subsequent event              
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]              
Dividends declared per common share (in usd per share)   $ 0.3645          
Scenario forecast              
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]              
Dividends, common stock $ 86.3