Income Taxes - Tax Cuts and Jobs Act (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Tax Cuts And Jobs Act Of 2017, change in tax rate, provisional income tax expense		$ 136.7
Tax Cuts And Jobs Act Of 2017, provisional income tax expense (benefit)	$ 657.8	644.7
Tax Cuts And Jobs Act Of 2017, change in tax rate, deferred tax asset		13.1
Tax Cuts And Jobs Act Of 2017, provisional income tax expense (benefit)	$ 0.06
Tax Cuts And Jobs Act Of 2017, effect of change on earnings per share, diluted (in usd per share)	$ 0.05
Tax Cuts And Jobs Act Of 2017, provisional amount recorded for valuation allowance release		$ 36.4